ok maybe u say how much money u have. If u hav more than $5000, then u may buy the fish vitamins that costs $180 each.
Then if there are any sick fish they should be fine for quite a while cos' the red liquid make them stronger and stronger.(of course u still need to cure them)
And since u said u have all mage fish except one, u should be able to thrive soon. Anyway u didnt say what kind of fish/fishes are sick often.
Is it the magic fishes/normal fish?