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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13 Responses to Experimenting with Tap Water for Opae’ula Tanks

  1. Shrimp Head says:

    Just had a comment on using tap water to top off water that evaporates – the only problem I see with this is that tap water no matter how clean will contain some dissolved minerals/salts etc. In and of themselves it would not be an issue, but as time passes and water keeps evaporating, since there’s no other way for anything to leave the tank other than pure water (evaporation), the minerals and salts are left in your tank and slowly builds in concentration. At some point this concentration of whatever minerals (copper in the pipes, minerals from the aquifer) might start negatively affecting the shrimp. If you do regular water changes the effect is somewhat mitigated, but from my understanding with Opae Ula most people don’t do water changes, so there’s a real risk of having non-harmful minerals build to harmful concentrations.


    • JimS says:

      Hi, Shrimp Head. Good point. And it’s something I was concerned about, too. But I decided to go ahead anyway for a number of reasons: (1) The bottled water I was using also contained minerals. (2) Minerals are in rainwater. (3) More minerals are picked up by the water as it seeps into the aquifers or flows toward the ocean. (4) The minerals in the aquifers are considered safe for human consumption since they’re in drinking water. (5) These minerals end up in the natural brackish water anchialine pools that are the opaeula’s natural habitat. (6) Dennis Nakashima, an early pioneer in sustaining opaeula colonies, uses tap water in his thriving tanks. And (7) my early experiments were successful.

      Still, I would advise caution in switching to tap water for the very concerns that you raise — as well as others. First and foremost, check with your local board of water supply to make sure that they’re not adding chloramine or other chemicals, harmful to the opae, that can’t be easily removed. Next, do it with a small experimental tank first. Do it gradually. Watch. Observe. Assess. If the opae continue to thrive or appear to be even healthier, then continue the test over a period of time that you consider safe. There’s no guarantee that tap water will work for you, so this might be a risky experiment. -Jim


    • JimS says:

      Re salt accumulating over time — that won’t happen if you’re topping off with fresh water. The amount of sea salt remains constant. A minute amount may be lost via salt creep due to evaporation, but adjustments in the tank setup can solve this problem. -Jim


  2. Shrimp Head says:

    The natural anchialine pools are not closed system by which water can only leave by evaporation. Hence the minerals would not necessarily build up continually – there’s another exit route by which they can potentially be carried out via tidal movements through the interconnection with the ocean.
    Rain water falling straight from the sky wouldn’t have minerals – minus whatever is in atmospheric dust that dissolves in the clouds and as the rain falls – rain water is essentially distilled water. Once it falls to the ground it will of course dissolve minerals.

    It all depends on what type of tap water you have and the level of minerals you have in it. For example, in Edmonton Alberta where they have hard water (most taps have a crusty mineral build up) I am almost certain you will run into issues using that water to top off an opae ula tank.

    It’s funny that you mention salt – it’s a particularly illustrative example of how evaporation concentrates whatever is dissolved in water. You wouldn’t top off an opae ula tank with salt water at the same salinity precisely because evaporation will start concentrating the salinity pretty quickly to full seawater and beyond…now imagine any other mineral that’s in tap water – it might take a longer period of time because they are at much lower concentrations, but the same thing is happening. At some point, unless if you take it out of the system by doing a water change – the concentrations will keep increasing. And maybe it will take years for that to happen – when the calcium/magnesium/phosphorous/potassium levels finally reach a point where it affects the shrimp. Or maybe they get happier and happier – I have no definitive answer at this point, only educated guesses.

    Long story short – just wanted people to understand the potential risks, and that unless if a particular tap water is tested for a long period of time, it’s hard to know whether or not it will be safe for the long term, and given the potential lifespan of opae ula, the relevant time frame is long indeed.


    • JimS says:

      Re rain and particles, review condensation nuclei.

      Re anchialine pools’ connectivity to sea water, review hydraulic conductivity.

      Re deteriorating water quality in a tank: the primary causes are opaeula waste products and decaying food. In our (Nakashima’s and my) case, we don’t feed the tanks, and we rely on the biological filtration of UGFs to manage the waste.

      Water hardness (mainly calcium and magnesium) in tap water, which you’ve mentioned, is not an issue in tanks.

      Re cautioning readers re experimenting with tap water, please review my earlier post. -Jim


  3. Shrimp Head says:

    OK, I’ll bite – your typical condensation nucleus is 0.0001mm. A typical raindrop is 2mm diameter. If the CCN is completely soluble, say 100% salt, that works out to a concentration by weight of 0.27125 parts per TRILLION. I say atmospheric dust/pollution will likely have a greater effect on any dissolved solids than the condensation nuclei.

    I don’t see what hydraulic conductivity has any relevance to what I’m referring to…my point is precisely that there’s another route for water/minerals to move away from and into the pool. My point is exactly that the hydraulic conductivity through your glass tank is ZERO.

    So you are telling me that GH is not an issue in tanks, especially in a situation where you are continually increasing hardness through evaporation and no water change? High GH can affect the ability of shrimp to molt, and while the Opae ula are very adaptable animals, I have to imagine at some point the GH concentration will become problematic…of course this is again assuming you do no water changes and keep topping up with tap water that has measureable GH.

    My point on experimenting is that potentially effects won’t show up for a while. My relatively well covered fish tank loses roughly 5% water volume per week. So if this was an opae ula tank being topped with tap water at 6dGH, and I top up every week, with no water changes, in one year my dGH will be at 21.6. In two years it will be at 37.2. If dGH at 21.6 is not an issue for Opae Ula, I suspect that at 37.2 it will be. As per my point – to truly assess the impact of topping off using tap water is a multi year experiment. In that time span I can say that very few people who have read this page will come back to give an update on their experimental results. Even if you tell people to experiment – this is potentially a multiyear effect that won’t show up until they think it’s successful…

    Of course everything resets once you do a full water change.


    • JimS says:

      Hint: My previous message was in response to your previous message. Thus, view my responses in that context. In that context, extrapolate the info I’m referencing to the comments you’re making about rain and anchialine pools.


      • Shrimp Head says:

        What extrapolation? I mentioned rain water not having minerals and you suggest that a condensation nucleus of 0.0001mm adds enough minerals to be significant? Fact is that while perhaps the same minerals end up in an anchialine pool, it doesn’t have the same concentration effect which a brackish tank faces when topped off with tap water.


    • JimS says:

      Re your August 29, 2018 at 7:41 pm message: Apples and oranges. The obvious fallacy is comparing a fish tank to an opae’ula tank. How long have you been raising opaeula?


  4. JimS says:

    I’d still like to know how long you’ve been raising opaeula. If you’ve been successfully raising them for a few years and your tank(s) are active and breeding, then I’d be interested in learning about your setup.

    Re physics — I mentioned the sources earlier because the physics in some of your pivotal comments is shaky.

    If in fact you have little or no experience with opaeula, then I’d invite you to take up the hobby to see if you can create as natural a habitat as possible to sustain these little guys indefinitely. They’re long-lived (20+ years) and fun to watch.

    You’re right re long-term effects. But we can still learn a lot about them in short-term experiments. If you’ve been raising opaeula, you’d know that you could lose an entire colony in a few days if the environment is somehow degraded. You won’t have to wait years. Also, if conditions are right, they’ll be active and multiply quickly. Again, you won’t have to wait years. Months and sometimes weeks will bear results.

    The gold standard is a minimal-maintenance tank with no mechanical filtration, feeding, or water changes that sustains an opaeula colony indefinitely. (By “minimal-maintenance,” I mean tapwater top-offs only.) The person who comes closest to attaining this goal is Dennis Nakashima, with one of his tanks. His videos are on YouTube, and they’re inspiring. His approach is unorthodox and pushes the envelope re what we think we know about raising opaeula.

    I haven’t been able to attain this minimal-maintenance goal yet. I’ve tried a few times but gave up when I saw the colonies dying off. I’m still relying on a UGF.

    I’m not claiming that I have all the answers. All I’m doing is experimenting and observing, basing the wisdom of my practices on observable outcomes. For me, there are two tell-tale signs of a healthy tank: the opaeula are active and there are signs of ongoing breeding — sometimes heavy, sometimes light. When activity decreases and the population seems to be dropping with no signs of breeding, then the alarm in my head sounds. I make changes, and partial water changes are an option. For one small tank, I did a complete water change. Again, this trial and error process doesn’t require years. I can sometimes get feedback in days. The key is to experiment with small tanks and a handful of opae. The opae are hardy, so I haven’t lost an entire colony yet. They respond to adjustments and can recover fully.

    There’s still much to learn about these amazing creatures.


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