Oh, btw, how is the copper/stuff inside the fertilizer ok for the fish? Why does the aquasafe "neutralize" it when I add water to my tank, if it's not really harmful to fish (im guessing its not since im adding it for the plants.)
Of the various mineral micro-nutrients required by aquatic plants, there is iron, copper, zinc, nickel and manganese. These minerals are also known as heavy metals because at high enough levels (this depends upon the mineral) they are toxic to all organisms. Plants and fish will be harmed by an excess of any of them, and if sever enough will be killed outright. Obviously, our goal is to provide the minimal level required by plants, which will not harm them or the fish. We also need copper, it is found in multi-vitamin & mineral supplements; but not in sufficient amounts to kill us.
Many medications are copper based, esp those for parasites like ich. Several warn the aquarist to use at half-strength with sensitive fish and invertebrates, and some believe that plants can be harmed as well. Some fish have very low levels of tolerance; Corydoras catfish and all the characins are particularly adverse to copper, and when it is used these fish usually shown clear signs of stress--rapid respiration, sometimes even gasping air, and lethargy leading to death if the copper is not removed via a water change.
Heavy metals are present in tap water in most areas of North America, in varying degrees. That is why water conditioners detoxify heavy metals. Plants also detoxify heavy metals. "Aquatic plants readily take up heavy metals," writes Diana Walstad, a microbiologist; as she explains, this is not merely as an assimilation of these minerals as nutrients, but rather the plants actually remove the heavy metal from the water and make it ineffective, i.e., detoxifying it just like the water conditioner does. This is yet another benefit of live plants in aquaria; fish are bound to be healthier--but I digress.
There is of course a limit as to how much of these heavy metals the plants can detoxify, just as there is a limit to how much the water conditioner can detoxify. Seachem told me that Prime only works on trace amounts as is normally present in tap water; in excess, Prime is completely ineffective at detoxifying heavy metals. Fortunately, especially so since fish can tolerate far lower levels of some like copper than humans, our water boards regulate the water supply to maintain healthy levels, or so we hope, and the plants and the water conditioner will handle what is present.
In summation, the plants must have these micro-nutrients, and to some extent an excess will be taken up by the plants and detoxified. However, rather than try to figure out the level at which this will no longer work, it is safer not to overdose fertilizers. Long before the mineral may be toxic to fish, some will affect plants negatively if they are at a greater level than what the plant needs as nutrient. This is why I always recommend a balanced comprehensive fertilizer, one that provides the correct amounts in proportion, according to what the plants require; it avoids accidental overdosing that could lead to fish and plant health problems. This is one of the major issues I have with the EI (Estimated Index) method which, in simple terms, means dosing certain mineral nutrients into the tank at levels greater than what the plants can possibly use, then removing the excess with massive water changes before trouble ensues. I really do not see the logic in this method. But I'm digressing again...
Hope this helps.