Hmmm, getting a better sense. The 300gph was going in, not out. Do an experiment. See how long it takes to fill a gallon jug from your reactor output, then recalculate for head pressure if you plumb it higher.
If you don't mind powerheads for in-tank flow...then I think no big deal if you only get 150-200gph from return into frag tank. Its only 20g and it doesn't take much to turn over that volume.
What are you using for an overflow? Perhaps post that for help with sizing a return pump. Remember you'll want a constant water level in your skimmer compartment....or if not using a compartment then make sure turning off your return pump will not overflow your skimmer and collection cup.
I was surprised by the damage I did to an old mj impeller where there was some rubber around the shaft tip...think it was rubber. But I let that sit for days by accident. I'm just saying be smart as vinegar is more potent than you might otherwise expect. I think all the rubber is visible...I was overly cautious about my power cord and the impeller this time is all. A short soak isn't likely an issue for anything
I put about a gallon of water in and splashed maybe a cup of vinegar...then dropped in my powerhead last night. I yanked the pump and propellar after 20m or so, but otherwise left the components in all night that had no rubber. Coralline flaked off when I ran tap water over the pieces that sat overnight. I had to really scrub the pieces I didn't soak as long, but vinegar seems to really eat up some rubber given a past experience.
^ Jealous...as you can probably pull that off with this size tank and enough rocks/pods...where I will have to wait until I get a bigger tank.
What kind of lights are you planning on? How are you going to dose? If you need 2-part or dosers then jump on the BRS group buy