As said there's 100 µF on the input of the HydroMonitor.
No other caps (none on the input).
No parts missing.
Pumps as said rated 3-4W so that makes 250-300 mA each.
So the question remains: is it normal for bushless pumps to mess up the power supply? I can't find detailed info on this on Google, and I don't have (access to) a scope.
The simple solution is of course to just use a separate power supply but it's; not all too practical.