Honestly I think you just need more filtration more importantly higher turnover to keep up with bio load as been suggested byI am using the API one.View attachment 1390562
On your tank I would be shooting for minimum 1500 gph turnover with factoring in head height, the fluval filter is rated for 900gph Z zero head height once you add 5-6feet your lucky if your getting 300-400 gph. I wouldn’t add multiple canisters, a sump would be the best bet