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Is 2 fluval 407s a good idea for a 90 gallon tank. One on each end with cross crossed intakes/outakes. No big fish, not massively stocked, medium level of planting.
I would want about 4kg of biomedia to filter 90 gallons. 2 x 407s gives 8 baskets total. Each basket holds about 1kg. If you had sponge in 1 basket on each filter that leaves 6 baskets that potentially could hold 6kg of biomedia. So i would be happy with that as filtration. 2 x 307s with 6 baskets total would also do the job, with 4 baskets left for biomedia.