However good your water quality may be in the beginning, it will deteriorate after you put fish in the tank. Fish create waste, plants lose stems, and there will be uneaten pieces of food. This detritus is decomposed by bacteria through the processes called the nitrogen cycle. The first stage of the nitrogen cycle is extremely toxic. Ammonia is excreted by fish and also passed into the water by the decomposition of organic compounds such as plants is extremely toxic to fish and can burn their gills.
Whilst you may be able to save a fish with only small signs of this damage, likely the results will be death either in a day or two or much earlier than it otherwise would have been. This is converted to nitrites which are still toxic, and then processed into nitrates. Nitrates are relatively harmless, but sudden or long term exposure can still be dangerous to your fish.
When it comes to fish tanks, you generally get no chances. Yes, there are some fish species such as Siamese Fighting Fish, mud skippers and some other breeds that can tolerate higher levels of nitrates and ammonia than others but left too long any tank will turn into a fish graveyard. Often a tank getting dangerously high in Nitrates or Ammonia appears more or less the same as a tank with perfect water conditions so don;t wait until your water has turned green from algae feeding of the nutrients in the water…. that happens days after the water quality has turned. You need to put in appropriate filtration to suit the size of your tank, the amount of type of fish in your tank and depending on whether your fish are cold water, warm water of salt water fish
Sponge filters can help combat nutrient levels in the water. Sponge Filters consists of a perforated plastic tube that is fitted with a cylindrical sponge. Air goes through the tube, and draws water into the sponge. These filters are mechanical and biological, and are best suited to aquariums with low filtration needs.
Protein skimmers are another option. They are essential to a marine aquarium. They use a process that is called air stripping. Organic materials and proteins stick to air bubbles, which form foam that rises to the top of the tank and is collected in a removable container which is emptied daily. These filters do not work in fresh water.