Breeding Opae’ula in a One-Gallon Tank

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Experimenting with Tap Water for Opae’ula Tanks

Buying cases of bottled water for the brackish water mix in my small tanks has been such a hassle that I’m taking a serious look at tap water. Dennis Nakashima uses tap water, apparently without any problems.

My nephew, a hydraulic engineer with a private firm, has worked with the Honolulu Board of Water Supply on a number of occasions and is familiar with our water quality. He says that the BWS does add chlorine to our tap water in Honolulu to control bacteria — but not chloramine. This is good news because chlorine gas can be removed easily but chloramine is a different story.

Other parts of Oahu and Neighbor Islands may rely on different chemicals, so it’s best to check. My nephew provided further information:

Click on the link below to get the water quality report for you property:

Once you get to the website click on:

Search for and download the 2016 Water Quality Report for your address

Type in your street address (no need to include “Street”).

I’ve read that leaving water out in an open container overnight will remove chlorine gas. There are other natural methods, including leaving the open container in direct sunlight, swishing the water around with your hand, pouring from a higher distance. I could boil the water, but that seems inefficient. I could purchase a water filtering system, but that seems expensive when maintenance is factored in. Mechanically, I could run an airline from a pump into the water to speed up the dechlorination. I could also add chemicals, including vitamin-C, but I don’t want to take the risks involved.

Leaving the water out overnight in a 3-to-4 gallon open tub seemed to be the easiest, most natural, and least risky method. I then poured the water into empty plastic water bottles for storage.

In this process, it’s important to remember to be very careful when using buckets, bottles, funnels, etc. that have been washed with soap. Be sure to rinse thoroughly to completely remove all residue.

The Water Cycle,” Board of Water Supply.

I also realize that tap water could contain other natural chemicals that might impact water quality. However, Honolulu’s water supply seems to be very clean:

Most of Honolulu’s consumers get their fresh water from the island’s extensive aquifer systems…. Aquifers are permeable rock formations from which fresh water can be drawn. In some cases … some of the wells are artesian which means … this water … has taken hundreds or millions of years to filter down to the aquifer rock.1

Thus, I’m hoping that our island’s natural filtering system has removed most if not all the potentially harmful chemicals.

The Water Cycle,” Board of Water Supply.

There’s also the possibility that the plastic containers that I’m using might somehow add chemicals to the water. I need more info about this.

This morning, I topped off the 18- and 1-gallon tanks with the dechlorinated tap water. I’ll continue using bottled water for the 10- and 1.5-gallon tanks. If this experiment proves successful, I’ll switch completely to cleaned tap water.

1 Larry Kobayashi, “What Is the Current State of Fresh Water Supplies in Honolulu and Oahu: Will We Have Enough Water for the Future?” Hawaii First Water, LLC, 12 Sep. 2014.

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Question from John in Rochester, NY

Last updated 6/4/17 12:17am


John in Rochester, NY
Submitted 4/28/17

I’ve been following your blog for a while now and have set up an Opae Ula tank using your ideas.

My tank is a “tall” 16 gallon (same footprint as a 10 gallon tank) and I have an under-gravel filter, under about 2 inches of black pea gravel, with two small power-heads up toward the top of the tank. I filled the tank to the top of the glass. I don’t like the looks of a partially filled tank. I bought a “tall” tank thinking that if the output from the power-heads and sponge filter are far-enough away from the gravel below, the Opae Ula won’t experience much turbulence below. I bought those very-low-flow power-heads set on the lowest flow setting. The output from the power-heads is 12 inches from the gravel. I’m also using an air-powered double-sponge filter that I have set to release the bubbles just below the top surface of the water. There doesn’t seem to be much turbulence at the lower level of the tank. I’ve had my Opae Ula just two days, but they are all hiding. Do you think I have too much turbulence, even though the power-head output is set to the lowest, and the bubbles from the sponge filter and the power-head outputs are all happening a full 12 inches from the gravel surface? Hmm… I’d actually like to SEE these Opae Ula… Maybe I just have to be patient…

My tank parameters seem OK, ZERO ammonia and nitrite and 20 nitrate… I cycled my tank for almost 4 months before ordering them. There is green algae on the glass, on the pile of coral rock I have and on the large lava rock I have in the tank. I have an LED light that I have on 14 hrs per day. I’m not using a heater.

I’m hoping the power-heads work out… Or maybe I could create a “pre-filter” in the under-gravel filter riser-tubes using some filter floss? I could attach some tell-tails on the power-head exhaust tubes to monitor output flow… Putting some floss in the riser tubes would beneficially slow things down coming out the power-heads…

I like your idea of a drip mech filter… I may have to ditch the power-heads and connect the under-gravel filter to some kind of mech drip arrangement if my Opae Ula remain too shy to come out…

Keep up your very interesting and valuable research and your enjoyable-to-read blog.

John in Rochester, NY


John, first, thank you for the kind words and my apologies for the delay in responding. A combination of work and question marks with the drip filter system I’m now using has kept me from blogging.

I agree that a partially filled tank looks weird. My 18-gallon is similar to yours (10-gal footprint) but remains only half-filled as an ongoing experiment in avoiding salt creep. Now that the AquaLifter system seems to be controlling creep, I’m keeping the water level at the 10-gallon mark. I’m not sure if added depth will improve the overall environment.

A practical consideration is water quality maintenance. The bigger the tank, the more water I have to deal with, and 18 gallons is a lot to have to mix and maintain, especially since I’m using bottled water. (I’ll be experimenting with tap water dechlorinated by overnight standing. I learned that the water supply in Honolulu is treated with chlorine but not chloramine. A huge difference because the latter is difficult to neutralize.)

Your experimentation with low-power powerheads and sponge filters is also very interesting. Please keep us posted on your observations.

My experiences alone aren’t enough to form valid generalizations. We need as much input from as many opae’ula hobbyists as possible. Thus, your observations are invaluable. Having said this, I’ll share my thoughts.

I’ve been experimenting with the AquaLifter drip filter system for three months now and believe that, in its present configuration, it works best for my tank parameters. All my tanks use this system, from 1-gallon to 10. One of the 10-gallons uses only one pump, like the 1-gallon, and both seem to be fine.

I believe the best filtration system is one with a very slow flow rate. In their natural habitat, anchialine pools, water flow is imperceptible. Water from fresh and sea sources pass through permeable coral and lava rock formations before reaching the pools. The filter systems that I tried in the past were all too turbulent. By contrast, the AquaLifter is rated at 3 gallons per hour. It’s literally a tiny stream of water that barely causes a ripple on the surface. I haven’t found anything comparable anywhere.

The problem, though, is that the AquaLifter + prefilter was not designed for this use. It’s used primarily in sump systems to manage overflow. I had to experiment with different configurations before I got it to work right. It’s fine now, but aesthetically, it still looks jury-rigged. Hopefully someone will come up with a clean and efficient out-of-tank pump-filtration design with a very low flow rate that also looks good.

I check the pumps at least once a day for any signs of leaking. So far, the pump-on-high setup seems to work best. When I had it lower, lifting and pushing water from and back into the tank strained the tubing at the joints and caused leaks. Placed above the tank, half the pressure work is eliminated by gravity and there have been no leaks.

A recurring problem, though, is algae growth in the airline tubing. I know when it’s time to clean them when the flow begins to stutter into drips rather than a continuous flow. So far, it’s been about once a month. Cleaning is simple. I run a long bamboo skewer into the tubes — from both ends for longer tubes — to loosen the algae. Next running tap water through them or, better yet, blowing through one end, will spit out the gunk. This only takes a few minutes. In all cases thus far, the flow was immediately back to normal.

You mention inactivity in your tank. My best guess is that this is a problem. Opae’ula are curious and playful little guys, and they love to explore and swim about. That’s 24/7, even at night when all lights are off. A sign of a healthy tank, I believe, is activity. The more the better. Still, the inactivity you observed was only a few days after releasing them into the tank, so in time they may have become more comfortable and active.

If inactivity is still a problem, you might want to turn off the powerhead and filters to see what happens. If they come out of hiding, then you’ll know turbulence may be the problem. Unlike fish, opae’ula can survive without mechanical filtration for long periods of time (days), so don’t worry about turning it off.

I experimented with sponge filters for a while. However, they had a negative impact on my tanks. In time, they seem to turn the habitat into a wasteland. I’m guessing that they’re filtering out good as well as bad bacteria. But this is my experience, and YMMV. I removed them from all my tanks.

Lighting, too, could affect activity. Opae’ula are known to shy away from bright direct light. You might want to experiment with lesser intensities. Late at night or early in the morning, with all the lights off, check for tank activity with a small, narrow-beam flashlight. If they’re active, then it may be a sign that your daytime lighting is too intense. A solution may be to move the tank to a place where it receives indirect light or brief periods of direct sunlight.

I’m not sure how your gravel and rocks are set up, but the thing to keep in mind is their natural habitat, anchialine pools. They are not like coral reefs or salt- and freshwater fish habitats. It includes both hypogeal (subterranean) and epigeal (surface) regions, and they are active in both. The hypogeal is best seen as a network of subterranean tunnels and caves created by the gaps between coral rocks.

I’ve been relying on Dennis Nakashima’s codominium coral mound for hypogeal simulation, but for my small (1-2 gallon) tanks, it wasn’t that effective. For these, I’m using a strata design, with the hypogeal forming a full layer over the gravel bed. This leaves a wider epigeal layer above for free swimming. This works so well in the smaller tanks (active in both, breeding in the larger of the two) that I’m thinking about doing the same with my 10-gallon tanks. The problem, though, is that creating a hypogeal layer of coral will probably require a lot more coral, adding weight to the tank.

If you have plants — I’m not certain, but I don’t think plants are an important part of their habitat — at least the kinds of plants that we normally see in aquariums.

If you have other inhabitants such as different types of shrimp, snails, etc. — their natural pools are usually limited to opae’ula only and not shared with other creatures.

Re filter material in the riser tubes to slow flow — an ingenious idea! That could work. If you try it, please let us know what happens. It would serve the double purpose of also cleaning the water. One drawback may be that the filter could restrict flow to the point where the powerhead will stop working. Also, the filter could quickly become a source of contamination if it’s not cleaned often.

One of the reasons I moved to an external filter system is that a UGF system ultimately relies on a gravel filter that reaches a point of contamination that could be dangerous if water is not changed periodically. I think this was happening in my 10-gallon tanks, where activity was diminishing over time. When the original filter material in the AquaLifter prefilter gets dirty, I’ve been replacing it with activated carbon pellets. This way I avoid having to continually buy prefilters, which could add up quickly.

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Zoeae in the 10-gallon Tank

I guess the surest sign that the 10-gallon is recovering is berried females and zoeae. I’ve seen both in the last two days. In this video, you can see two zoeae drifting up and down in the tank. I’m assuming that the switch to the gentler Aqualift pump system is making the difference. This tank has only one pump+filter unit. The 18-gallon has two. To make sure that both halves of the UGF are filtered, I simply take the output airline from one exhaust tube and place it in the other once or twice a month.

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Aqualifter Pump + Filter: Routine Maintenance

Airline Tubes

Check the drip rate at least once a day. Any slow-down may mean a problem with one or more tubes:

  1. Look for crimps in the airline. Replace if you find any.
  2. Look for signs of algae build-up within the tube. If you notice a small clump of growth, remove the tube and blow into one end. A quick burst of air will force the algae out.

Aqualifter Pump and Prefilter

The prefilter sponge turns dark after 2 to 3 months. I tried cleaning it, but the sponge material seems to lose its porousness and puts a strain on the pump, causing a leak. Instead of purchasing new prefilters, which could quickly add up in cost, I’ve decided to replace the sponge with carbon.

The problem is figuring out how to force the water entering the filter to pass through the carbon pellets before exiting and returning to the tank. To this end, I cut a small rectangular piece of plastic to fit inside the filter to keep the carbon section separated from the exhaust section. The theory is that most of the water entering the filter has to pass through the carbon before it can seep below and around the plastic barrier.

The plastic material I’m using for the barrier, however, is too thin and bends under the weight of the carbon, defeating the purpose of the barrier in the first place. I’ll have to come up with a stiffer barrier or a different design.

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Aqualifter Pump+Filter Change for Small Tanks

The plastic mesh didn’t work out. (See “Aqualifted 1.5 and 1.0 Gallon Tanks” and “Oblong 1.5g Tank with Aqua Lifter Pump.”) Salt creep on the mesh was bad. Decided to use the same configuration as the larger 10-gallon tanks, with the pump-filter unit in a small container above and close to the tank. (See “10-gallon with Aqualifter+Filter – Updated 3/27/17.”) So far, no salt creep problem with the pump-filter units on the 10-gallons. This unit, however, doesn’t have a drip-back safety system (see below). In case of a leak in the pump-filter unit, the water will fill the container and spill over onto the shelf. I’ll have to think of way to allow for spill-back. The top of the tank is too narrow to support the pump-filter unit.

This is the one-gallon. The pump-filter unit looks huge on this tiny tank. Note the small hole in the middle of the white container, about a half inch from the bottom. This is a drip-back safety system. If the pump-filter unit leaks, the water will drip back into the tank. This is the same system I’m using with the larger (10-gallon) tanks.

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10-gallon with Aqualifter+Filter – Updated 3/27/17

This is the 10-gallon on the kitchen counter with an Aqualifter+filter on top of the tank. I removed the plastic spacer from the glass canopy. I also removed the lighting to make room for the pump+filter unit. The slots were already part of the small plastic tray. These will serve as an overspill if there’s a leak, sending the water back into the tank instead of onto the counter and floor. Shot with X100-T with poor in-door lighting at 10:30pm. Grainy but does the job. I’ll shoot it again during the day. Settings f/2, 1/15 sec, ISO-6400.

Updated photo 3/27/17. Photo taken 3/27/17, 8:23am, f/2.5, 1/50 sec, ISO 800. Slightly smaller aperture, a lot faster shutter speed, and a much lower ISO. As an informal test, I’m using only one pump+filter on the 10-gallon and two on the 18-gallon below. The logic for two is that the UGF is split into two halves, so each side needs a unit. The logic for one is that the water in the unpumped half will seep into the pumped half and circulate. I didn’t create any openings between the two halves in the UGF so this assumption may be wrong. When it becomes clear that one is enough or two are required, I’ll make some changes.

New photo 3/27/17: 18-gallon (filled to 10-level) shot 3/27/17, 8:22am, with 2 Aqualifter+filter units. What you can’t see is the opening at the back of the canopy, similar to the one in the photos above. You also can’t see the small hole drilled into each tray to serve as an overspill back into the tank. To accommodate the Aqualifter units, I removed the back spacer from the canopy as well as the light. Photo specs: f/2.8, 1/80 sec, ISO 800.

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