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Help!

I bought a red oscar (about 1.5 - 2 inches) at the weekend and added him along with 2 algae eaters and 2 clown loaches to my tank (48*24*18).  The following day we had a MASSIVE nitrate spike (from around 5 ppm the week before to around 40 ppm) although ammonia and nitrite remained 0.

We immediately did a 50% water change and repeated it yesterday and today.

Nitrates are now down to between 5-10 ppm.

The problem is that the new Oscar has developed a pea-sized lump on his side near his tail.  It started out as a small shiny patch of scales which looked like natural colouration. This morning it was a slightly bigger shiny patch but still looked natural, but this evening it has developed into a nasty looking whitish-grey lump with what looks like an almost bony protrusion.

He is still swimming about fine and coming up for food, and if anything, is even more active as he settles into his new home.

I've added Melafix in the meantime on the assumption that it's probably a bacterial infection due to the nitrate spike but really that is just a guess.

Is there anything else I can do to help the wee guy?

(I'd add a photo but I cant work out how!)

Some of these lumps come and go.. AS you just got him?? you might want to try some Parasite meds.. NOW as you have scaless fish here, the pleco/loachs, MANY meds can irratate them to possibley kill them so be careful here on which med U use.. this sounds like a worm cycst to me.. AS it is opening up.. I would either get another tank, LIKE a 10 gal to treat him in, or take him back for another one in better condition.. they contact these worms from a contaminated host fish.. AS you have a new tank here, that is kind of hard to do.. UNLESS the carrier is the pleco or the loaches....

IF you feel that you can treat him and want to try, then get the 10 gal tank, a small filter, heater airstone OR use one line from the main tank if it is a two line Air pump.. ( I do this for smaller temp tanks).. The best med for this would have to be ONE of these: Furan 2, OR furanase, OR maracyn plus.. BEST to use another tank as some meds may kill out your good bio in the tank.. AND that you dont' want.. Follow direction on the meds..