hay buddy one thing i have learnt from building live bait tanks u can never have a big enough overflow fittings i have 60mm outlet 19mm inlet and it still fills up quick.. 500lph pump ps if u are gona use a big skin fitting make sure u mesh the inside cause yakkas can get stuck and cause tank to overflow,
also make sure the water runs down hill otherwise u will have a trap of water which will make the tank overflow, (water cant get out) i have seen many people do this..
good luck make looking forward to some pics!!