I currently have a 50 gallon tank, I just upgraded to a filstar xp3/Large. I wonder if it would hurt to add a Eheim Classic 2215 that I picked up used. Or would this be too much flow for a moderately planted aquarium?
I run the XP3 on several 45 gallon tanks, and I have added power heads for more water movement. Mostly the Koralia 4 (over 1000 gph). These are the older Koralias, when they first came out and only had about 4 models. I know the model numbers have changed.
I have a 2217 and a 2213 on my 50 gal, and it's not enough flow. I have a Koralia 240 helping with circulation, but there are still dead/slow spots. If you can afford it, go big and dial it back to where you need it.
I'm running two Fluval 405's on my 50g (36x18x18). The flow is fine and I actually have it scaled back quite a bit. One filter runs through a pvc reactor and then through an inline heater and then out of a spray bar. The other filter just runs directly to the tank and has the standard output nozzle.
I have xp3 on my 75, I don't know why you would want another filter the xp3 is awesome maybe ad a power head
If all you're concerned with is flow, then yes, add a circ. pump. If you want the added benefit of another filter + greater circulation, then add another filter. Adding the second filter was one of the best moves I've made with my aquarium and I wish I had done it sooner. It's good to be able to alternate maintenance between the two filters. It's also good to have a back-up in case the main filter goes down- you're not left without filtration.
I'm not super familiar with rena or eheim, but I can tell you that it's nice to have two of the same filter. All of the parts and components are easily interchangeable. I don't have to buy components for two different filters.
I ended up putting the Eheim on my wife's cichlid tank. I bought a Fluval 306 on sale from PetSolutions ($96) ordered it Thursday late at night, it arrived Saturday morning (fedex). I put it on my tank with the xp3, flow is perfect, and it never hurts to have some redundancy.....
By the way I'm pretty impressed with both filters, the Fluval seems like a pretty solid addition.