Before commenting briefly on some misconceptions mentioned since my last post on water changes...to the question of fertilizer harming your tetras, generally no unless they are overdosed to excess. However, a basic concept in an aquarium with fish is to limit all additives to only those that are essential, regardless of whether or not they may be directly harmful to fish. We must remember that water--and with it, all the substances we add to the water--is continually entering the fish primarily by osmosis through the cells, and this water is then processed by the kidneys, enters the bloodstream, affects other internal organs, and so forth. Fish are not exposed to any of these chemical substances in their habitat, so their physiology has to deal with them as "foreign substances." While this may not immediately kill the fish, it has the likely effects of weakening it, causing stress, and perhaps shortening the lifespan. All because the fish has to somehow react and deal with these substances. So, it is better to keep them to the absolute minimum.
I recently had a problem in one of my tanks, and after months of tests and experiments with on-going advice from some of the most knowledgeable individuals in this hobby, including microbiologists and biologists, I got to the bottom of it. But along the way I learned that I was adding more plant additives than necessary, and these were in fact contributing to the issue. I try to learn from my mistakes, and I have since reduced the additives by more than 3/4 with no injurious result to the plants (actually the opposite, interestingly) and I know the fish would thank me if they could. And in a sense, by their recovery and behaviours, they have done so.
Which leads me to the misconceptions about water changes. Diana Walstad's method requires minimal fish load and heavy plant load, and that sets up a different environment than what most members here have in their aquaria. I know I certainly have more fish in my tanks than she does, probably by two if not three times from the numbers I have read in her book and columns. Water changes removing plant nutrients is misleading, unless we have the data. Algae certainly is somewhat thwarted by more water changes, so it could be assumed that "nutrients" are being removed, but if so the question is just how much and how necessary for the plants were they to begin with. It is also possible that the water changes are having some other effect that thwarts algae.
There are still compounds entering the aquarium water continually that no bacteria and no filtration can remove. And the fact that fish have been shown to be more responsive (benefit) by more water changes cannot be argued. The long-term effect of all this is the real problem. The more water changed, the more stable the aquarium will be.
Byron.
