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That Darn Plant Tank:

text and photos by Erik Olson
December 1997

In parts 1-5, I went through the details of setting up our big 75 gallon plant tank, and described some of the plants we're keeping in it. This time around, I'll discuss two interesting ``spin-offs'' from the main tank.

[Diagram of Sump System]

The Daphnia Reactor

Here's the problem: As soon as we set up the new tank and moved all the plants into it, we got a major case of new tank syndrome. Actually, ``new light syndrome''. And time for green water to jump into competition with the plants.

Green water is a term used to describe unicellular algae. It's a problem in the tank because fish can't eat it. Usually it's due to high nutrient level (nitrates, etc.), but I think in our case it was the sudden change in light, coupled with the plants' shock at being replanted in new substrate. It was so bad that for the first week I could not see from front to back of the tank.

Typical methods for dealing with green algae include starving it, keeping the tank in total darkness for several days. Another method is to use a very fine filter, such as a diatom unit or micron cartridge filter. I didn't have the luxury of either, since my tank was next to a window in the middle of summer, and I had just spent all sorts of money on fishtank stuff anyway.

There is one situation in which one wants to have green water: raising Daphnia. Daphnia are little water crustaceans, often referred to as ``water fleas''. Some call them the perfect live food, because of their low fat and high protein content. They're pretty small (up to 1mm in diameter), so they're ideal for some baby fish. And they eat small suspended particles in the water, such as unicellular algae. A popular way to raise Daphnia is to take a big tank of old tank water and leave it out in the sun for several weeks until it gets infested with green water, and then seed the tank with Daphnia until they have consumed the algae.

So, how to do it here? I couldn't keep them in the main tank; the fish would immediately make short work of them. I'd read in The Aquatic Gardener how Paul Krombholz kept Daphnia in a ``cage'', a sort-of floating fry hatchery net thing with very fine netting. I could do something like that. In fact, for my tank I could do it one better because of all the elaborate plumbing already in place!

I took a ``short'' pickle bucket (about 3 gallons), drilled a hole in the side, and fitted it with some PVC gear to make a crude overflow. This ran into the sump through some vinyl tubing. The overflow was covered with filter sponge on the inside of the bucket. Finally, I ran a slow flow off the same manifold used to drive the CO2 reactor (slow enough that it took maybe 10 minutes to fill up the 3 gallon bucket).

From there it was a simple matter of adding a starter culture of Daphnia to the bucket. They didn't escape the bucket, because of the sponge and low flow. But they consumed the green water that entered the system, just like a filter or reactor. In fact, in under a month the Daphnia population exploded and the water had become clear. And then they effectively died out because they had no more food. A few baby fish, dumped into the reactor, feasted on the remaining few before I disconnected the reactor.

Problem solved!

The Triangle

Something I've always wanted to try is a ``Micro Plant Tank''. In Amano's Nature Aquarium World, Volume II, he has some beautiful setups that are in 10 gallons or less.

I've had nothing but bad luck with this myself. The main problem for me is first, justifying the expense of another CO2 system for a tiny tank. Another problem is instability of such a small system. I kept a tank in my office, but could never keep much in terms of great plants; the water evaporated quickly, the temperature fluctuated with the heat of the lighting above the tank, and the chemistry drifted a lot. And third, the filter and heater took up too much space!

The desire for a tiny tank was intensified as I watched my Kribs totally destroy a small patch of Glossostigma I was growing in the big tank. That did it.

I hit on the idea of running a micro tank off the same system as the 75 gallon, just like the Daphnia reactor. I took my failed micro-tank home from work, and set about re-plumbing it. Two holes were drilled in the side, one for an overflow and one for a return. That was all! I hooked in two vinyl hoses to the sump, and I had an instant tiny tank with all the benefits of a big tank... water reserves, CO2, heat.

Plants in there have done quite well. For several months, I had Glossostigma forming a pretty carpet. Micranthemum succeeded it, sending out runners that took over the tank and choked it of light, and stringy green algae choked it of nutrients. Though I wasn't able to save the Glossostigma, I was able to get rid of the brush algae by introducing the livebearer Ameca splendens into the tank. Three young fish were added, at which point we stopped adding any food. Their bellies are always full, and they are growing. And there is a little stringy green algae in places they can't reach, but for the most part it has been eradicated.


Perhaps the most important ingredient in keeping a successful tank has not been mentioned in the series thus far: patience and attention. It's now been a year and a half since we set up the tank. In that time, it's grown from pea-green soup into a beautiful full-grown garden. Then, of course, last summer my now-wife and I ended up moving just as I had alluded to in earlier parts, and it went back to an algae-choked thing for a while as we dealt with more ``important'' matters... painting, moving boxes, getting the bathroom to work, cleaning the other 17 tanks of spawning dwarf cichlids. Finally, after some water tests and appropriate adjustments to the operations (more carbonates, cranking up the carbon dioxide level, replacing some of the old lights, and fertilizing regularly again), the tank has responded in kind by exploding with new growth. Guess it just needed to be noticed!

That's the end. I also have some later E-mail discussions/clarifications, or go back to the index.
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