Only suggestion that i have after further review is to maybe drill a small hole in the top of the submerged elbows to prevent airlock, and not to solvent weld (dryfit only) the interior pieces inside the drums. Also, I don't know if you've used uniseals before, but make sure that you deburr the holes, or you'll get leaks. To get the pipe through the seal you're generally going to need some kind of lubricant (soap is probably OK for the waste drum, but not sure what a reef safe option for the RODI and salt water barrels might be. spit?) Just kind of make circles while applying pressure until the pipe penetrates the seal. Good luck and good job on the design! I probably would have used more pumps to further isolate the RODI water from the salt water (ie another pump in the RODI water, and dump water into the top of the salt barrel), but I tend to overkill on just about everything.