Thank you both very much, for your replies! Carlos, the closest pet shop to me is a PetSmart, so I will bring some test water there. I prefer the smaller shops, but I suppose it will have to do. I used a little tap water to clean the filter, but I left the tank water untouched as the tank itself had been established for a while.

Geoff--that article looks promising, too. However, it seems I'm not allowed to read the full text (which likely entails treatment) unless I buy a subscription to an online library. Do you know how to administer the ivermectin?