Your local OATA retailer should be able to help you with general advice on fish health and there are plenty of treatments you can buy off-the-shelf.
But there may be times when you need more specific help. Again your local fish shop may be able to advise on local vets willing to treat fish or you can contact the Fish Veterinary Society.
The Fish Veterinary Society is for those with specific interests in the health and welfare of fish, whether farmed, in public aquaria or in the ornamental sector including fish kept as pets. The Society holds a list of vets who are willing to treat fish (in both commercial and domestic situations).