Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Here is how I handled the situation, and how it evolved. Not saying it is The Recipe, just what worked for me. I hope you can find some sort of hint here that might be a starting point for your system. Every set up is different. Your tap water might have more minerals so you do not have to add trace nutrients. You might have so few fish that fish food just about does not count as fertilizer...
Few plants, low light, gravel substrate, big fish load. Fish food was the only fertilizer for a short time.
Plants showed potassium deficiency.
Same set up. Added Leaf Zone (Potassium and iron). Plants looked better, and in ways that made me sure they also had some iron deficiency before.
Same set up, but more plants. Added Excel. This was even better, but the plants started growing faster to the point that they were removing more and more of the nitrogen, suggesting that they might be also using up the other nutrients common in fish food.
Hint: if you still have to do water changes to keep the NO3 low, there is probably enough other nutrients (P, traces) for the plants, but you may have to dose K, C and Fe.
Phase 4: I noticed the plants nearest the windows were growing faster than the others, so I doubled the lighting. I also started using better plant substrates, less gravel.
At this point the plants needed a lot more fertilizer, so I got into the EI method. I was still used to doing pretty large water changes on most of the tanks anyway, to keep the NO3 low so that part of the EI was fine. I still had a large fish load in most tanks, so I was already fertilizing the tanks that way. Now I was adding straight ferts as well. I found I did not have to add very much NO3 to all the tanks, but some were using more, so I had to test to see I was not adding too much. Similarly, I did not have to add Epsom salt or GH booster to any tank. Some fertilizer recipes include Epsom salt as a magnesium source- well, the GH was stable in my tanks, suggesting the water changes were bringing in enough CA and Mg.
I backed off the full EI method to what worked for me. While I was still heavily fertilizing the high CEC substrates had picked up a lot of fertilizer so now I am much more casual about how much fertilizer and how often. The substrate is a cushion that keeps the plants going when I do not fertilize. I do a lot fewer water changes, and most are around 25%. I have smaller fish loads in each tank.
Hint: I still use the NO3 test to determine fertilizer needs. I have practiced enough that I know how my tanks respond. On those tanks where the fish food is not enough I add enough of all the fertilizers in the ratios that work for me. On tanks where the NO3 holds pretty stable (plants are using it about as fast as the fish convert fish food to fertilizer) I dose less of most ferts, and little to no NO3. More K and traces.