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.