
According to your measurements....(I supposed it was inches, though cm would make a difference) I plugged them into the aquarium calculator at the top of the page under "ARTICLES"....which gave me 18 us gallons, 15 uk gallons.....the rule of thumb is 1" of 'adult' size fish per gallon of tank. This is measured on what the fish WILL grow to, not what they are as babies. The fish profiles also under ARTICLES will help you figure the expected growth.
We always advise people to only purchase fish that will not out grow the tank they are going into.....not what most folks do, but it does cut down on fish deaths and emergency tank purchases....
I am glad your fish is doing better, the bruising should heal and if you can get the fungus under control, scales do grow back. You could use a medication that stimulates the slime coat, like maracyn-plus which places the medication right in contact with the fish, and locks it there with a product that is similar to the natural mucous coat of a fish.
As for 40 nitrates, this is the reason I suggest upping the water changes. 40 is stressful to fish, and will lead to fin rot, popeye and all sorts of other fish diseases....even if you have 10 nitrates right out of the tap, that is MUCH better than 40 and will bring down the overall readings with consistent every other day changes, until you can divide the fish. Good Luck
