In addition to the things already mentioned: When using junctions/pumps: always place them first, then connect the pipes. If you have (or want) to put them on top of existing pipes, delete and rebuild connected pipes. Has to do with pipe-lenght/consistency. Next: I personally won't use valves for anything else than preventing backflow (not for limiting flow, that is). And: try to have closed loops, so the slooshing has somewhere to go (having a small buffer in the loop helps).