i was trying to get a mag 12 for my tank but online it is 13 dollars more to get a mag 18. what it be better to get the mag 18 and dial it down but if increased flow is required later i will have it or should i just get the mag 12. its a 180 with 2 corner overflows one overflow has the return and the stand pipe and the other has nothing in it right now

personally id go with the 18..
i did pretty much the same thing, i got the 24, and throttled it (redirected flow) until now when i use all of it.. :)

the more flow the better, i always say. i am going to use a mag 18 on a closed loop, on my 125 with 8 ball valve controlled outlets. you should be fine on a 180.