would it be better to have 1 25mm return from the sump or 2 20mm? its for my anemone tank and im stuck on what to do

Many people just have one but you may want two if you are wanting to use them to get flow where you want it.

No safety issue involved I've never heard of a return blocking.

i was thinking of having 2 returns to get flow on both sides of the overflow but thought maybe i might have issues with flow to both returns as im running a 1060

One is going to cost half the price. just so long as the one you put will handle the flow.

of course having a second hole, means you can have/switch to full syphon system and have a back up.

The other thing to consider when running multiple pumps is heat. Every pump you add is in effect a heater.

BSG

Sorry I misread thought you were talking about two overflows. Two returns is a dumb idea (IMHO!). It's not needed and you would have twice as many things to go wrong, if you want the output going in two places use a SWCD or 'T' the return.

Personally I wouldn't use a SCWD on the return as they do reduce flow quite a bit, but a simple T would be pretty effective.

one return, match piping to pump outlet, split it into 2 just before it hits the tank (reduce pipes but NOT to half the original) work it out to be same or slightly larger, pi r^2

thats how i was going to do it chimera just have 1 return pump and t the pipe into 2 outlets, chimera if i use 32mm pipe for the return what size should the 2 outlets be?? i hate maths!

32mm = an area of 804mm squared. If you were to T it off, then you need 25mm on each (25mm = an area of 490mm squared, multiply that by 2 outlets = 980mm squared) So you'll lose slightly more pressure coming out of the 2 outlets compared to a single 32mm. Probably so little its not worth worrying about as you gain the advantage of water movement at each end.

(and just as an example, the next size down from 25mm piping is 20mm. If you went to 2 x 20mm outlets then this is only 630mm squared, 174mm less area than the outlet of the pump!)

thanks for that i thought that having 2 20mm would br better but ive changed my mind now