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That Darn Plant Tank: A Word About Fertilizers

February 1998

When I first published this set of articles for our club's bulletin, I was so burned out on how long it took to describe construction of each of the tank parts, that I was happy to finish up everything in part 5. Unfortunately, I totally neglected to say anything about my fertilization methods! So here, better late than never, is a few words about fertilizers used in That Darn Plant Tank.

This tank followed a very Dupla-like approach for fertilization, a combination of substrate and water column fertilizers.

The substrate was approximately 170 pounds of #3 "texas grit", with laterite mixed in with the bottom third. Laterite is a reddish clay from the tropics first marketed by Dupla (at prices exceeding several dollars per pound), but also available from other sources. In the past, I have used laterite-like clay from Alabama and Lake Shasta. The laterite for this tank was bought from an art clay supplier; As a result, I was able to obtain about 50 pounds of it for about $30. I have been happily passing on bags of it to everyone who has been interested locally, and still see no end of it in site.

In my previous tank, I beleive I overdid it in terms of the ratio: too much laterite. I chose to mix in about 700 grams this time (10 grams per "gallon of water"). There are several dangers with mixing it too richly. The first is that it can compact, forming overly-anaerobic areas, and preventing the circulating effect desired from the heating coils. The second danger is that too much can leach into the water column, turning it red. No matter how little you use, it will initially redden the water for a time. This will also happen every time plants are pulled up. However, I have found that these trace amounts quickly settle out in the established tank, leaving it quite clear.

The other half of the fertilizer is the 2-part Dupla regimen of tablet fertilizer at water changes, plus daily drops. The theory here is that the tablet contains trace elements which are not toxic in large abundance, and so can be depleted until the next water change, while the liquid drops contain nutrients that either are naturally extinguished within 24 hours or are toxic in large quantities. A large portion of DuplaPlant 24 drops consist of iron.

Why did I skimp on the laterite, but spend $40 per year on dupla fertilizers instead of rolling my own (ala PMDD)? It wasn't the money; I wanted to try the alternative laterite, and it was there. Someday I will play with rolling my own liquid fertilizer, but for now it seems like a lot of work with testing levels, something I don't want to spend the time on.

One last note: Follow the regimen, it works! I inadvertantly did a small experiment to prove this when we moved to our new house (see part 6). For several months, I just didn't take the time to dose the tank with the daily drops. I still added tablets at the occasional water change, and the lights continued to automatically turn on every day, but the plants were undernourished and small. Algae was plentiful. After I returned to regular daily routine, plants began improving within a week.

Next: In part 6, I'll wrap up with some ``spin-off'' tanks that we built after the main tank, and how the big planted tank is doing now, over a year and a half after we set it up.
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