I have 2 5inch oscars in a 55 gallon tank. My tiger oscars scales are falling off and it seems his tail is almost see-thru what could be wrong. My Nitrate level is through the roof (way over 40) on the test strips. I'm new at this so bear with me. How much water do i need to change and how often. Please Help.

firstly, you need to think about either getting a bigger tank, or getting rid of one of your Oscars, 55 gallons is not big enough for two Oscars. Unfortunately this is a prime example if you overstock an aquarium. I'm not actually having a go at you, I'm just using it as an example.

Okay, a photograph would be very helpful. The problem with keeping Oscars in a small environment is nitrate levels will build up very quickly, they need plenty of water to live in. How much are you feeding each day? I would suggest that you actually reduce the amount of food you put into the tank, this will help reduce the amount of waste the Oscars produce. Remember, the more you feed Oscars, the more they go to the toilet, this will then affect how much nitrate builds up. Reducing nitrate is done by water changes, you obviously aren't doing enough water changes your nitrate levels are well above 40 ppm.

It's difficult to advise you regarding tank maintenance when you obviously have too many fish. Like I have already said, my first advice would be to reduce the amount of fish you've got.in the meantime, you need to increase your tank maintenance so that your nitrate levels are no higher than 20 ppm. ideally, You might want to keep your nitrate levels lower than 20 ppm, unfortunately, in your situation I think this would be a lot to ask for.

Regarding what might be wrong with your Oscar, really not sure until I can have a look at the photo. However, in most cases, poor water conditions do have a lot to do with diseases, illnesses and ailments in fish. There is a possibility that your fish may have fin rot, this is often caused by poor water conditions. You mentioned something about scales falling off,difficult to say, I've seen fish with this condition before, sometimes it is caused by a parasitic infection of the skin, may be a fungal or bacterial infections, I couldn't possibly say.

I'm sorry if a lot of what has said is not what you want to hear, unfortunately for a lot of people, they don't realise that looking after an Oscar and keeping it healthy is not quite the same as looking after a few tetras

The first thing you need to do is get the nitrates down and as OFL said you do this by doing water changes. I would do a 50% water change then do daily ones of 20% until you nitrates are much lower. Also usually you should only feed what they will eat in 2 mins, i would half this until everything is sorted out

Can you post a pic.

Is there any cotton wool type stuff on the fish, are the edges of the fins missing, are the fish fighting?

55g is too small for 2 full sized oscars so either get a bigger tank or rehome one

I still have my 3 oscars in my 55 gallon. I have been cycling my 160 for about 8 weeks just havent had a chance to switch their tank. I never have problems they are 7" but I have (2) 75 gallon an hour HOB filters and (1) 125 gph cannister filled with biological. I guess it depends on your setup?