i have leech in one of my tanks but they seem to be harmless and just eat small snails. (they are small and brown not the big black gross things) at first i was very panicked and looked at a lot of info to get rid of them. the meds that kill them are illegal as far a s i can tell and the only other way that seemed to work for people was to evacuate the fish to a new home. can you seed the filter with mature media? yours may be leech infected. will the lfs give you any? then people were ditching their plants and chucking loads of salt on everything, stripping the tank down, boiling everything and starting again... theres lots of instruction online if you google it. or you could pick out what you see and hope for the best? others suggested leaving liver in the tank to attract them but you would have to monitor this very closely so it didnt foul the water... you need to look into medicating your fish in their hosp tank too if they have parasites... this is all worst case senario, maybe someone on here has experienced it and can guide you. good luck