i don't know anything about the
ferplast cayman 230ltr tank with bluwave07 internal bio and mechanical filter at 900l/h
but if your tank is 230liters and you're only pumping 900liters of water through an hour you are going to want more powerheads. most people aim for a minimum of 10x's the volume (230 liters means you want a minimum of 2300 liters per hour). depending on what you want to put in there you may need less (not likely unless you are looking at seahorses) or more. I think i have some where around 15x's, some people go even higher than that. good luck
what power heads do you reccommed and where to position them. also what about skimmers. i have 100mm or 4" behind my tank. which one best to get. thanks
The best powerheads to get are tunze streams but they are very expensive, there are a cheaper version called the seio superflow pump which ar ebrilliant too and very affordable.
The best hang on skimmer is the deltec mce600, around £110 ish 2nd hand on ebay, dont get a cheaper as you will probably wasting your money