If I have some substance that I want to get inside of my fish, but which is something that can't be simply fed to them...either because it will be lost in the water, or they won't eat it, or whatever other reason...I have had great success with a couple of different methods.
First, if you happen to feed your fish any type of gel food, either commercial or simply DIY stuff, it is very simple to add the supplement to the gel during mixing. A properly-mixed gelatin food dissolves very slowly in the tank, so if your fish have anything resembling an appetite, the stuff will be eaten before any is lost. With larger fish, you can be quite accurate with the dosage simply by doing some math to create a mix that contains a known quantity of supplement, divide that amount into individual portions that contain the amount you need the fish to eat, and then cut that into small enough pieces so that you can add them by hand, one at a time, making sure that each fish gets the correct amount.
For larger fish, it's even easier to buy empty gelatin capsules at the drug store and then fill them with the appropriate quantity of supplement, then slit open a larger piece of food and tuck them inside before feeding. This works great with things like whole frozen fish, fresh earthworms, etc. Jam the capsule into the food item, from front to back, as a predator normally swallows food headfirst and this keeps the capsule from being squeezed out during swallowing. Again, this requires that the food item be swallowed whole.
Either method works well with powdered supplements. For liquids, it works fairly well to apply the correct number of drops to a couple of freeze-dried krill or a partial cube of freeze-dried tubifex, and feeding this after the liquid has been completely absorbed. Freeze-dried foods soak up liquid like a sponge. This isn't as accurate in terms of dosage as the other methods, so maybe not ideal for medicines but great for vitamin supplements, etc.
