Water quality goes beyond just filters and often involves human intervention like doing water changes and monitoring water parameters for fish tanks to stay in equilibrium. As was said bio load, plant mass, types of plants you have, gas exchange rates, water volume, light and CO2 levels, and bacteria levels all effect this equilibrium and how much intervention is needed. I have no doubt that a tank can be kept without a filter system especially on larger tanks but in general filters only help with all of those things listed above. Some will cite maintenance as a con but when I think of all the gunk that I remove during a 3 month service, I'm glade that I can easily remove it and start with a fresh filter. Filters also significantly improve the bacteria levels for the tank increasing the chances that water quality will remain very good.
Do bettas need filters? - Fish - Hamster Hideout Forum
Sucker fish like a well-decorated tank. Although they may interact with other fish in the tank, a pleco is most comfortable with plenty of places to hide. Intricate ceramic aquarium decorations that tend to collect algae are practical if you have a sucker fish in the tank; he'll keep them polished. A tank that receives natural sunlight through a window for part of the day and consequently grows algae will supply the sucker fish's need for fresh greenery and also give him a chance to bask for short periods of time. Add at least one piece of driftwood to the tank; sucker fish like to chew on wood with the rasp-like teeth that surround their mouths. Keep water temperature in the same range as for other tropical fish (68 to 82 degrees F) and be sure the water is moving and aerated by using filters.
Why You Need An Aquarium Filter | petMD
I disagree with changing your filter. If you are using a chemical filter (like carbon), then you’ll need to replace that monthly. But mechanical and biological filters don’t need to be replaced until they are physically falling apart. Your filter media is where beneficial nitrifying bacteria lives (this converts ammonia and nitrite, which are harmful to fish, into nitrates, which are easily removed with weekly water changes). Every time you replace your filter media, you are destroying bacterial colonies that are important to the health of your tank. Instead of replacing your filter media, just give it a swish in a bucket of tank water after a water change. This will keep your filter from clogging without removing those beneficial bacterias.
Is a filter necessary for fish tanks? - Quora