Depending on how quickly the fish are losing weight will determine what the problem is likely to be. If the fish have taken a month or more to lose weight then it is probably intestinal worms, (tapeworms or threadworms). If the fish are losing weight quickly, within a week or two, then it is probably an internal infection like Hexamita.
The fact that it seems to be spreading from tank to tank, and the catfish has stringy white poo, would probably indicate an internal bacterial or protozoan infection. Metronidazole (Flagyl) should work on this type of problem but the medication will need to be used for at least a week, preferably 2 weeks. It will also wipe out the filter bacteria so you will need to do daily water changes to keep the ammonia levels down. Tolak should be able to give you an accurate dose rate for this medication.
If you started using Clout and it appears to be working, you should continue treating the fish until they are cured, or as often as the directions state. If you stop treatment before the fish are cured, the disease can come back and be harder to cure. You are also more likely to cause drug resistant organisms because any pathogens that have been exposed to the medication and survived, will pass their immunity to the drug, onto their offspring, (when they divide or reproduce).
Any fish with the problem will probably have released bacterial/ parasitic spores in the tank they were kept in. Therefore you should either treat the tanks as well as the fish. Or move the fish into a clean tank for treatment and disinfect the other tanks.
A mild bleach solution (not sure on dose rates but about 10% is common) can be used to disinfect the tanks. It is a good idea to drain the tank and refill it several times before bleaching. You also want to gravel clean the tanks and clean the filters. Then wipe down the inside of the tanks to get rid of the algae and slime on the glass, rocks and ornaments. The less gunk in the tank the more effectively the bleach will work.
Tapeworms can be treated with Praziquantel. Use 100mg of Praziquantel per 20litres of water. Do a water change 24hours later. Then repeat the treatment a week later.
Levamisole Hydrochloride can be used to treat Camallanus threadworms. Depending on the concentration of Levamisole will determine the dose rate. Camallanus will show up as little red hair like things hanging out the fish's bum.
Other intestinal worms need different medications, most of which we can't get in Australia without a vet prescription. Flubenol is available in the states and treats all intestinal worms but I’m not sure if it’s available in Oz.
If you are losing fish from the problem, then take one of the sickest ones you have to a fish vet or gov run animal health lab and get the fish autopsied. They will be able to tell you what is causing the problem and give you an appropriate treatment.
