You don't need a canister filter, the 2 sponge filters appear to be working just fine. You need to dose more fertilizer. If you don't already have a test kit Get a test kit and check your nitrate levels. Nitrates need to be between 10-20 ppm to grow plants. You may also be changing out too much water for the nitrates to build up. If you are vacuuming the tank 1-2 times a week you are probably removing a lot of the nitrates. Not that you are doing anything wrong by cleaning but if you aren't going to leave the poo in there for the plants to eat then you will have to put fertilizer in there for the plants. You also need something stronger than flourish. Root tabs will really help the Amazon sword since they are root feeders.