+1 i use two filters on my moderately planted 40 gallon. They are both rated for 70 gallons. The tank stays nice and clean. The plants are growibg and the fish are healthy and active.It doesn't take much surface agitation to cause a gas exchange. I have been using an airstone in a tank with no filter (temporary fix I had to do), but as soon as I add a filter of any size that airstone will be turned off for the benefit of the plants. Filter ratings are like opinions... They are out there for your consideration, but they may not apply to your individual situation. Plant density is a major factor on how much filtration is needed to keep the tank healthy. Some people even have densely planted tanks with no filter at all! There does have to be water movement of some sort unless the fish are surface breathers like Bettas.
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