Thoughts on Filtration in Opae’ula Tanks

Updated 11/26/15, 11/27/15, 2/19/18

A couple days ago, Odin asked, “What are your views on using an air filter in the Opae Ula tanks?”

Good question, Odin, and like all good questions, it’s tough to answer. I guess it all boils down to your goals.

Among hobbyists and casual researchers, at one end are the purists, who want to replicate the Nishijima bottle experiment. For them, mechanical filters are out of the question. The test is to see if fluctuations in temperature during the day can create enough circulation in a tank to cycle harmful bacteria into good bacteria. The goal here is a sustainable ecosystem that requires little or no maintenance.

I haven’t actually tried this, but based on Dr. Wayne Nishijima’s and Dennis Nakashima’s experiments, this natural filtration system works. In Nakashima’s case, the tanks are much larger, and he set them up with a stratum substrate of tiny shell fragments and a sloping pile of small coral rocks on one end. This configuration, he assumed, would facilitate the natural circulation caused by temperature changes.

Nakashima also kept his tanks in locations where they received some direct sunlight. His assumption seems to have been that the additional light would improve circulation and increase algae growth — algae being the opae’ula’s primary food source. His assumptions appear to have been correct. His colonies have not only survived, but they’ve thrived, with continuous breeding and increasing population growth over many years. And this is with little or no maintenance. 

Interestingly, Nakashima also experiments with under-gravel filters (UGFs) using powerheads with good results.

I’m not a purist — at least not yet. I’m part of the group that wants to create an environment that emulates the opae’ula’s natural habitat, anchialine pools, for the purpose of observing their daily activities, especially breeding — but without having to wait a year or longer. According to Nakashima, it was a year before his colony began to breed.

I’m not that patient. Thus, I looked around to see what others have done to speed up the process. The one thing that jumped out at me was the fact that commercial breeders seem to rely on mechanical filtration, usually systems designed for fish but jury-rigged to accommodate the tiny opae’ula.

I decided to go with filters, too. For my first opae’ula tank, I used a 5-gallon Fluval Chi, which came with an external filtering system that sucked water out and pumped it back in. This setup didn’t work out for me, with waste and sediment accumulating on and in the gravel stratum substrate and a constant fear of opae being sucked into the filter.

I switched to sponge filters instead. These kept the water clean, but they also seemed to be removing good bacteria, turning my tanks into semi-sterile wastelands. I’m not sure if this is a valid observation and would welcome other’s others’ views.

I also worried about the danger these filters posed when I tried to clean the tanks. I was afraid that my arm brushing against the tubing would cause the filter to tip and slide, crushing the opae close by. I also worried about their burrowing into the filter core and becoming trapped.

Thus, when I decided to get serious with this hobby, I opted for a 10-gallon tank with UGF. That tank, begun a year ago on 19 Nov. 2014, is my most successful tank to date. (See the video above.)

With UGFs, I can actually see the water cycling continuously through the gravel, which serves as a natural filter.

UGFs also don’t pose the same threats as external and sponge filters. Opae can’t be sucked through the gravel into the chamber below. However, when I experimented with a stratum substrate of tiny shell fragments, I found that the opae are are able to and will burrow through a thick layer to get to the subchamber below the gravel. Returning to the surface, however, isn’t always as easy, and they become trapped. Thus, I use gravel exclusively.

I found that the opae aren’t able to crawl down the uptake or exhaust tube as long as the pump is on. This isn’t a problem since I leave it on 24/7.

A problem with UGFs may be turbulence. Many are alarmed when they see the strong flow of water being pumped back into the tank. In my smaller 1-gallon and 2-gallon tanks, I reduced the flow via a weaker pump, in one case, and a check valve in the airline in the other.

I haven’t tried to reduce the flow in the larger tanks since the colonies appear to be thriving as is. Breeding and healthy activity in the 10-gallon are ongoing. The 18-gallon, too, seems to be coming along fine. (The 18-gallon is actually an 18-gallon tank that’s filled to about the 10-gallon level. The remaining space acts as a buffer to reduce or eliminate salt crusting along the top edges and the canopy of the tank.)

I’m learning as I go, so these thoughts on filtration are works in progress.

There are other goals for creating an opae’ula tank. For some, the primary goal is aesthetics. Their tanks are displays, part of the decor, and some of these are stunning. This doesn’t mean, though, that they’re not interested in breeding. In most cases, they are, but the ornamental aspect is most important.

On another note . . .

This past week, I decided to reduce the number of tanks by eliminating the two 5-gallons. I moved the colonies into the 18-gallon. Salt crusting had been an ongoing problem, and reducing the turbulence and lowering the water level didn’t seem to work.

Update 11/27/15: A couple months ago, I found a slow leak in the 2.5-gallon tank and used the small colony to start the Oblong 1.5-gallon tank. Thus, as of this date, I’m running 4 tanks: 10-gallon, 18-gallon, 1.5-gallon, and 1-gallon.

Update 2/19/18: After experimenting with slow-flowing AquaLifter pumps in all four tanks, I’m back to UGFs for the 10- and 18-gallon. The AquaLifter setup that I used couldn’t sustain the water quality. (For more on the AquaLifter experiments, type “aqualifter” into the search box for this site.) Seeing the benefits of a gentler water flow, I’ve decided to take the advice offered by others to run the UGF intermittently. (Roughly an hour or two a day.) Thus far, it seems to be working. Breeding in both tanks has been active. The population in the 10-gallon was reduced, I think, by the AquaLifter experiment. Along with the experiment, changing the placement of the tank on the counter also resulted in less light. I moved the tank back to its original location where it gets the most afternoon sunlight light. With the reversion to the UGF and better natural lighting, breeding in the 10-gallon has been explosive. (See the recent videos.) My guess is that the colony is attempting to repopulate. In the last few weeks, dozens of zoeae have been floating in the tank, and I’m also seeing more berried females. I’m still using the AquaLifter setup in the two smaller tanks. In the smallest, the one-gallon, when I noticed that the opae were less active, I decided to recycle about 60% of the water. I ran the water through activated carbon a couple times and poured it back in. The wash seems to have worked. The opae are active again. The healthiest tank, thus far, has been the oblong 1.5-gallon, which requires the least amount of maintenance. The opae remain active. In both small tanks, I have sighted a berried female. In all tanks, I’m now using tap water to top off for evaporation. (Search “tap water” in this blog for more on this.) I haven’t done any water changes for at least a year or longer. I haven’t fed the tanks for at least two years. I’ve cleaned the glass, at most, once a year to increase visibility.


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4 Responses to Thoughts on Filtration in Opae’ula Tanks

  1. namruso says:

    Aloha Jim,
    I have a 5.5gal tank that’s using an UGF that’s working quite nicely. I have it with black gravel and red cinder as the choice of condominiums. I have the filteration pumped by a 40-85 gph (Adjustable!) pump purchased for $8.50 at Petland in Kahala.

    I have another set up with 2 55 gallon tanks with UGF paired with the same specs as my 5.5gal.

    5.5gal has the flow rate of 40gph while the 55 gal tanks have 85gph.


  2. namruso says:

    Good topic on filteration. I feel that overall, UGF have had the best results for me.

    Initial set up:
    55 gal tank
    Duo sponge filter
    Air pump
    Black cinder (substrate)
    Big red cinder

    1) Opaeula are very evasive and curious; the hid within my black cinder substrate. -.-
    2) duo sponge filter could not keep up with the dirtiness of the water
    3) sponges needed cleaning; good bacteria get diminished.
    4) air pump created more evaporation without helping to regulate the temperature

    Current set up:
    55 gal tank
    Lee’s UGF
    Black gravel
    Big red cinder
    40-85GPH Submersible powerhead

    1) Good choice in changing out the black cinder to black gravel as Opaeula could have been caught in the exhaust pipe of powerhead.
    2) Water is much cleaner, and they seem to enjoy the small current.
    3) Powerhead set to 85GPH seems to have a observable effect to the water temperature and Opaeula activity in respect to it set to 40GPH
    4) Tank in direct sunlight seems to be doing okay.

    Indoor tank:
    5.5gal tank
    Lee’s UGF
    Black gravel
    Red cinder
    40-80GPH submersible powerhead
    40 Opaeula

    1) smaller tank and less condominiums = more activity
    2) powerhead set to 40GPH, pretty decent current, and they seem to enjoy.
    3) Opaeula have lost some color either due to molting or to environment change.


    • JimS says:

      Aloha Namruso. Wow! You’re way ahead of me.

      I haven’t paid much attention to water temperature, except when the direct sunlight is intense and prolonged. I run my hand over the outer surface of the glass as a rough gauge to test for overwarming. For the 1-gallon on my desk, I’ve created a shade out of cardboard and string that blocks most of the sun, allowing enough light in over the surface. The result simulates the view you get when you’re diving. But the desk is in an AC room so heat may not be a problem.

      Mahalo for the tip on powerheads. I’ll look into it at the Kahala Petland. I always assumed that they’d be too powerful for my small tanks. The largest is 18 gallons, but it’s filled only to the 10-gallon level. I didn’t realize you could adjust the output. The price is definitely right!

      Re color: It’s a mystery to me. A number of theories, and they’re all probably partially valid. Shipment or transfer from one environment to another causes trauma that affects color. Makes sense. Molting, definitely.

      My 10-gallon sits farthest away from direct sunlight, and the opae in this colony are the reddest. It receives some direct sunlight when the sun is on the way down in the west, just before dusk, but that’s limited to an hour at most. Thus, most of the sunlight is indirect. On rare overcast days, I turn on the light that sits on the canopy for a few hours. This tank appears to be the healthiest.

      The 18-gallon is about 4′ away from a large south-facing window and glass door so it’s exposed to mostly indirect but intense sunlight most of the day.

      Your observation is interesting: “smaller tank and less condominiums = more activity.” I’ve been curious, too, about the relationship between condo size and activity.

      The one thing I’ve learned is that condominiums are actually tough to build. They take a lot of planning, preparation, and time. The coral rocks have to be gathered from somewhere, which isn’t always easy. They also have to be carefully selected for size and shape. Next, they have to be “sterilized” and positioned, one at a time, in an effective pattern that will hold up over time and simulate the natural hypogeal stratum.

      I envy your experiments with huge 55-gallon tanks. I live in a condo, and the rules limit us to small portable tanks, so 10-gallon is about the max. I’d assume that the larger the tank, the greater the breeding rate.

      Mahalo for sharing your observations! You’re a tremendous source of information for this fascinating hobby. I hope you’ll drop in often to share what you’re learning.


  3. Robin says:

    I have a 2 1/2 gallon that has a 1 gallon sized UGF with a an older air pump orginally purposed for a 5 gallon. I use a skinny white airstone that produces finer bubbles and I keep the top of the uplift tub submersed. I use a fish gravel mix. I have a light with a 7 watt bulb that is on all day for the plants. It’s raised to keep it from over heating the tank. It’s all very low tech.

    I have done water changes because the decomp from a piece of cholla wood was making the water foamy. The opae ula love the wood so I did water changes rather than remove the wood. I have a piece of coral with a maze of tunnels inside it and a piece of grey lava rock with little ‘rooms’ on one side that I placed upside down. Currently I have marimo moss balls, java fern, and java moss in the tank.

    While there are opae ula always about, you never see them all at once. The coral and cholla wood are their favorite hanging spots. Just sitting here I can count 7 (out of 26) little guys hanging in the bottom of the cholla wood and there is always someone on the plants. I also have two nerites in the tank cruising around. The shrimp have grown noticeably. I feed them about once a week with either algae tabs or ‘for shrimp’ pellets. There is evidently enough food now in there that they are no longer swarming the food dish. The food gets eaten, just not a lot of jostling for it anymore. I do have problems with salt spray even with the lid only partially open and will be playing with a solution here in the future. Lowering the water level just isn’t a viable option for me. And, OH!, I do occasionally find opae ula surfing the bubbles! 🙂

    I could sit and watch the little guys for hours!


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