Ok so I might have gotten a bit overzealous with my filtration. I bought an eheim 2217 to filter my 20L. Now the flow is making the tank crazy even with a custom spraybar that I made. Currently I have the outflow limited by about 1/3 of a turn but I am afraid I am putting a lot of stress on the motor. Can I switch out the 2217 impeller with that of a 2215 to reduce the flow or would this be a bad idea?
Yes, you can, perfectly fine. Unfortunately, it will still be pretty high in flow. Here is a thread on it:
Eheim 2215 and 2217 facts and DIY performance mods.