The only thing ive found you have to be careful with when using two filters is the flow of water inside the tank, I have the spraybar from the canister spraying into the back wall as both the filters pumping into the tank made a current that was pushing my plants over. The more filtration the better, my tank is around 200L & I have 2000L/hr & the water is crystal clear. As for the overflowing issue, I'm not sure how its possible with a straw, I believe all the straw does is change the way the water flows into the feeder. I have had straws in mine for over 6 months with no probs, the straw is too small to stop any major flow.