Is this strange behaviour for a catfish?
I am very new to fish keeping. I have a 100ltr tank with an Aquamanta EFX 200 filter. Water is heated at 28 degrees. We have pure aquarium gel balls and a fluval sachet in the filter and do a 20ltr water change each week, adding 5ml of tap water conditioner. I don't have a water testing kit so get the water tested at an aquatic place near where we live. It is usually fine but we had a nitrate and ammonia peak about three weeks ago. I did a 50% water change and added the fluval to the filter then. The water was fine when tested this morning.
I have 4 guppies, 6 harlequins, 4 zebras all these are between 1 and 2cm long. 2 yellow dwarf gouramis which are about 3cm long, 3 little albino corydoras maybe 2cm long and 2 snails. we were given a iridescent shark catfish by a friend last week but I am not sure if it is ok. Its a beautiful looking fish but swims frantically backwards and forwards along the front of the tank almost as though its trying to find the way out! I haven't seen it eat anything at all. It will suddenly just sink to the bottom occasionally for a while as if it is going to die, then starts the whole procedure all over again.
Can anyone tell me if this is normal or is it stressed? I have no idea what to do. It is about 10cm long. Thank you in advance!