If you are losing fish due to ich, one treatment solution we use is Ich Attack, which uses a blend of herbal extracts to attack the protozoa, which causes ich. In addition to this, if you increase the water temperature higher than 85, than you will speed up the life cycle to a point where the organism cannot survive. Outside of natural oils like ich attack, we are unaware of any chemical treatments approved for use in food-fish.
The Ich Attack should not affect your plants, but the increased temperature might. In general, ich is challenging to survive, and the mortality rate of the fish will be high regardless of treatment; often, people sacrifice all of their fish.