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As a complete beginner I would like to ask if any one can help with a "problem" my sister in law has with the above catfish.
It is black and speckled ,approx 3inches long and said not to grow more than 7inches !!!! The problem ,it has started to go blotchey. It seems to have started when pieces of cucumber were added to the tank which he likes very much. The tank is approx 7 months old with community fish. The shop where we bought the tank and fish are unable to help us .
fingers crossed

Post a photo of the fish in question.
My hunch is you have a species capable of growing to 18 inches and color changes are signs of mood, disease or distress.
Post water test results, temperatures, tank size and list other inhabitants.
You must provide enough information in your questions so that knowledgeable fish keepers can help you with specific answers.

Further information .
Tank size 20 gall,Temp= 79*F or26*C, Water test taken last Saturday
Ammonia up, nitraites up,
Fish 2No Loach, 1No Sucker mouth catfish, 7No silver tip tetras, 7No Plackies, 3No Swordtails, 5 No Mollies

Up to beginning of last week the water tests were fine. Then one fish was found to be dead, the water was tested and we were told the ammonia was a little up and to go and do a two bucket water change. At the end of the week we had another water test and this time the ammonin was up and also the nitrates. We have been told to do a one bucket water change and leave for a week before doing another. The blotchy appearance of the catfish does vary from time to time but never goes away entirely. All the fish are approx 1.5 inches. except the catfish approx 3.5inches and five of the plackies which are baby's 1inch.

If you go by the 1inch per gallon, your tank is a bit overstocked, as 7 x 1.5 = 10.5, and you have 2 lots of 7, a few other groups.

Is "Plackies" actually meant to say "Platies" - similar fish to the swords minus the sword part?

Anyways, you need to figure out where the ammonia is from. Is it simply that the filter can't cope, or your filter bacteria has died? Is there something "dead" in the tank?

--
Mats

Yes I did mean Platies. We could not find out why the ammonia was high but when we had the water tested last Wednesday we were told that it was getting better and not to do a water change for at least another week. The catfish appears to be acting normally although is still blotchey. I am unable at the moment to get a good photo of him,but will keep trying.

We have since found out that the fish in question is a bristle nose catfish. He seems very healthy.

How old is the tank, and what kind of filter do you have?

Ammonia problems are usually caused by the following:

• * A new tank
  * Overfeeding
  * Poorly performing filter
  * Something dead in the tank

I would imagine your bristlenose is this one: . Does that look about right?

The other common suckermouth catfish is . Maybe it's this one?