Ok if you have just 1 oscars on its own you could just get way with 40g. But A little bigger would have been better. most people want more than 1 fish, Your oscar can reach 6" in 6months or a little bigger. and upto 14" in perfect conditions. most fish will reach 9" or 12" in a tank within a yr. the main problems are a fish this size can make a lot of mess and it becomes a fight between you changing the water weekly and the fish doing his best to make it dirty again. For your fish to be happy and to make you life fairly livable, 1 fish to 40g will work. he will grow fine and you will only need to change one 3rd of the water a week but will need to test wky too. you will also need a very good filter. for a tank of 40g i would say 120g filter or more. So thats one option.... If you want more fish, as you already know a bigger tank will be needed as soon as you start to add more fish or your work load will mean 2 or 3 water changes a wk. this will become too much work for you and at best you lose interest in your fish. you really need to decide how many fish you want to keep and buy a tank for this. If you are going to buy a new tank and it is oscars you want to keep, get the biggest you can afford and have room for. this will mean your work load is less(less water changes) or you can add a few more fish. If the size of your tank is limited to cost or room, so will the amount of the fish you can keep. if you already have other fish and depending on their size you could need a tank very soon. testing your water for ammonia,nitrites,nitrates will tell you this, and this is why most people ask for water tests before helping. For your fish to grow well and be healthy all you really need to do is keep the water in the right conditions for that fish.So 1 oscar in a 200g tank will mean you could get away with one 1/3water change a month. put 4 oscars in and again your back to wkly changes.
