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I've had my pair of for a bit over a year now and both have been extremely healthy until recently. In the last couple of weeks the smaller of the two (about 9 cm) has been rapidly losing weight, he (at least I think it's a he) is still eating, in fact he seems more hungry than usual, but he is literally wasting away! Aside from that he seems a bit listless, has clamped fins and his swimming is weak. Meanwhile the larger one (about 12cm, believed to be female) is doing just great.

The tank is about 45 L and heated to 26°C, there are no fish other than the two catfish, they have never fought and both get their fair share of food (they are fed on dried bloodworms and axolotl pellets). There are no snails or real plants (though there are plenty of fake ones). The only thing that has changed in the last few months is that I forgot to do their usual water change recently so they had gone 4-6 weeks without a water change, I did a 50% change today and will do several small changes over the next week. I don't have a test kit so the water parameters are unknown.

I suspect that the main problem is some kind of internal parasite or infection though the lack of water changes probably didn't help, but I am not 100% sure as I have never delt with anything similar before. For the moment I am concentrating on getting the water clean and trying some live food to fatten him up a bit, tomorrow I will go to the pet store and see if they have anything for internal parasites. I am tempted to put the larger one in a different tank for a while but my only other heated tank is occupied by bettas (not compatible, I've tried in the past). I can partition the tank but I would still risk exposing the bettas to the problem so this is not ideal. Has anyone had any similar problems? Any tips or advice?

I managed to get some photos so that you can see what I am talking about (or maybe spot something that I have missed).

top quality water is always the first priority for good fish health. If you can isolate this individual it would be better. try feeding tetra delica, (the brineshrimp or bloodworm, or daphnia in the jelly sachets, as these are nutritious & easily digested & can always be at hand if you can get a packet of sachets.

It seems a little strange to be effecting the individual now , after a year if it is internal parasites, as they would affect a small fish quickly.

Is the other out-competing it for food constantly?

Anyone else?

As ombry wrote he is still eating I wonder whether out-competing would happen. However, after eating food, it could be the fish needs it for movement - perhaps the fishes used to share shelter, but now the larger one has driven the other out - and without shelter it might be wasting away from stress.

Alternatively, the fish must have cought a desease, but than - why only 1 and not the other?

If the above suggestion appies, partitioning the tank - and providing lots of cover in both halves - might solve the problem

If it's still eating, I would consider medicating it's food with Metronidazole (if you can get it easily in Au. that is).

Two common intestinal super-infections are Ameobic infections and Hexamita (the suspected pathogen). Both of which are consistent with the wasting away you are seeing and both are treatable with Metronidazole, particularly if the fish is still feeding.

Relatively speaking, it is a low-toxicity treatment (I would treat both fish), is very effective IME, and has little risk of development of resistance factors*** (it's a Prodrug, the metabolites are the active agents.)

***= FYI: I am strongly against the use of true anti-biotics in ornamental fish...

Thanks for the responses, he is still alive but looking worse this morning. The little guy always gets his share of food and I've never seen them fighting or competing (though they could be fighting at night), but I've partitioned the tank anyway, there is heaps of cover throughout the tank (1/3 of it is taken up by a big cave, the rest has fake plants). Still doing water changes but the filter is off now because he has started getting stuck to it. I am off to the pet shop soon, I will ask about the food and meds mentioned and look into getting a slower filter.

erm

shutting down the filter won't help the other fishes, let alone the tank

if you just put it on now, you'll release a whole stinking wad of toxic water in the tank wich is caused by all the bacteria dying in there, it can kill your entire tank.....
you must clean it now thoroughly before switching it on again

also: getting a slower filter will be less effective on the tank, you'll lower the waterquality for the healthy fish.

it's hard, but if the fish is now so weak it gets sucked up by the filter I think you have to accept your losses. but not gamble the entire tank and the other healthy fish for it.........

Well, he seemed to be improving for a while but then he died ast night.

Fortunately his tankmate is still fine.