Huge Lump o n my Diamon Tetra

Question
Hi there, I am a bit worried about my Diamond Tetra.  It has a large lump on its side, probably about a centimetre in size, and protruding about the same amount.  It looks like a big dome.  Its also starting to show through/develop on its other side.  Im not sure what it is or how to treat it, he seems happy enough, swimming around fine and isnt off his food.  I think it came from a wound he had, which i thought healed ok, a week or two after that this lump started to devlop.  Any suggestins are much appreciated:)

The tank is around 10 gallons i think, has one angel fish, 2 guppys, 3 penquin tetra, a red nosed tetra and the diamond tetra, theres also a pleck bottom feeder in there, PH is fine (7.6), Nitrates are a bit high (100 - 250) but thats always beena problem with this tank as there are no real plants, tank is around a year and a half established.  Temp is around 27

Thanks!

Answer
Good afternoon, Steven, thank you for your question.

Yeeeikes! I don't think I've ever heard of nitrates being so high. Have you tested the water straight from your tap? It is likely high in nitrates also.

Nitrate toxicity causes all sort of things including problems with vitamin absorption and unexplained tumors. What you'll have to do is change some water ASAP. Do a 20% water change every day for a week and see if that helps with the lump. Nitrites out of your tap may be high, if they are over 20 ppm I would consider investing in an RO/DI (reverse osmosis/de-ionized) unit. You can also purchase a drum, and buy RO/DI water elsewhere for your water changes. RO/DI water would  be healthier for your fish and yourself in the long run, if your tap's nitrates test 20 ppm or higher.

Your temperature sounds fine, if a tad warm, try to bring it down by one degree if you can. Your angelfish is going to outgrow the 10 gallon, and your pleco (unless he is of the dwarf variety) is going to outgrow almost any tank. In the long run, look to rehome your angelfish to a 110 liter tank, minimum (about 29 gallons) and see if you can trade in your common pleco (if that's what you have, looks like this - http://www.aquahobby.com/gallery/e_pleco2.php) for a smaller algae eating fish such as a couple of Otoclinus or an Ancistrus. But first, get those nitrates straightened out!

Good luck,
Nicole