I have got a 8 inch tiger oscar in a 70 gallon tank for a month. Recently I suddenly found she got the red colour faded within one night. And she is more aggresive and tends to attack the other fishs. She is busy on moving gravels and building mountains of gravels. I am wondering why so strange things happen to her.Below are photos of her

original colour

current colour

Have you tested the water, ammonia, nitrite and nitrate?

That is a very dramatic change in colour, and ever seen it that much before. Sounds as if your fish has become sexually mature. It may be why she is going through all this behaviour. Even there isn't a male present, she may well lay eggs, if it is a she, it could well be a he. When Oscars reached sexual maturity, they often carry out the same sort of behaviour as they would do if they were going to breed. My Oscars would do it over a year before they ever laid eggs. As the colour changed back to normal or as it stayed.

Thank you very much for your help and so rich information about oscars.
Actually I never test the water for ammonia,nitrite and nitrate. What I only do is keeping a biological filter and a mechanical filter running in good conditions.And also the water changes are done regularly every week. Sometimes I test pH of the water in the tank,which is always ranging from 6.3 to 6.6. I think the bad water is not likely arributed to the colour change of my oscar because the other fish and another albino red oscar in the same tank remain quite healthy.

I noticed this oscar has got lots of scale falling off. I am not sure if it is a normal cycle for oscar's life ,especially in sexual maturity,or just the consequence of fighting.

I will keep eyes on my oscar and will let you know the dynamic changes in the future.

Testing the water occasionally for ammonia/nitrite and nitrates is vitally important. You may well think that your biological filter is okay. There are so many things that can go wrong, if you don't test your water, you won't know if problems arise. This is when fish start becoming ill or die. If your skin the started peeling off, you wouldn't start asking friends what the problem was, you would go to the doctors for tests, it's no different with a fish tank. The tests can indicate what can be causing problems with the fish. Has your fish stayed that colour. I have to say I've never see the Tiger Oscar that colour before.

There are literally hundreds of things that can go wrong with fish. I can tell you that most vets cannot help very much with fish. The biggest problem is it's very difficult to take a fish to a vet, even more difficult to actually examine it. It can be done by anaesthetising the fish but this is very expensive

Sorry for my ignorance,I will test the water for ammonia,nitrite and nitrate ASAP. My tiger oscar still stays that sort of colour but the other things are alright.Here shows my another oscar,albino red oscar.

Latest news: the water form my tank has been tested already in a laboratory, ammonia and nitrite are all below the limit,nitrate is very high.The standard is applicable for purified water.

Half a month has passed since the rapid colour change of my tiger oscar. Last night I found she has layed eggs on the rock in the tank.I am afraid the albino red oscar is also a famale because I didn't see her exhibiting any sexual activity during the spawning of tiger oscar.
Whether the spawning is attributed colour change of oscar is on consideration. I will be seeing what happen then.