I have been pondering this for a good while now, and I wonder if anyone else might agree with me, or have a good argument to convince me that I'm wrong. I have a rena xp3 and an emperor 400 on my 55g tank. I have 3 discus, 4 harlequin rasboras, a golden killie, a single cardinal tetra, (getting a dozen when the lfs gets them in) and a bamboo shrimp. I've wanted to switch out the emperor 400 for a secondary xp3, but I've never actually decided to do it because of how expensive the xp3 is. I love how quiet it is, and now that I have the spray bar facing the glass, the water current is very mild on that side of the tank. The emperor 400 is another story. The impeller is loud, the water cascades into the tank and is not the quietest thing, (even with the water at the correct height) and the current in front of the filter is kinda hefty... if I ever want to do co2, I'd lose most of it because of this filter. Now, I know surface agitation is important for gas exchange. That's why I got the emperor 400 in the first place. I pose this to any of you folks with discus and canister filters... do you have your spray bar aimed into the tank or at the back glass, and do you just do canister or do you have a combo like I do? Will the decreased surface agitation of 2 canisters aimed at the glass be a bad thing for my fish? Big al's has them for $90, and free shipping within the next 12 hours. (not sure when that offer started, but it ends in 12 hours) Let me know what you think? This isn't JUST aimed at discus owners either, anyone with plants, or honestly, any opinions at all will be greatly appreciated. Thanks. 
