If you do use carbon remember that it will have to be changed often, these are some suggestions which you may find helpful, switch off the lighting for about 48 hours, then after that reduce the lighting by half. I once had a dead fish in the tank this made the water cloudy. A regular water change of about 50% should help. I don't know what type of filtration you use but ensure that you clean the filter on a regular basis. these are all basic, but if you look after the water quality it will look after your fish. In the UK you can buy what is called PUREPOND by Evolution Aqua, these are little black jelly balls which in fact contain millions of live bacteria. These were first used in large ponds for the keeping of Koi etc, many people use them in their aquariums to obtain crystal clear water. I litre of purepond is ideal for 20,000 litres so you only have to drop about 10 balls in your tank. The result is remarkable, I am sure that this or something like it will be available in your area. The best thing about it is that you cannot overdose, when you no longer see the small black balls in your tank just drop 10 more balls in, it does not effect the PH or anything else. You can if you want put the balls in amongst one of your media's in your canister filter, but I prefer to put them direct into the tank. Hope you get your water problem sorted out soon.
