the bottom tank pumps water up to the top, and it drains back down through standpipes. the big blue things are matten filters... the standpipes are behind them so the water has to pass thru the mat to drain down to the next tank. these are also probably handy for keeping fish out of the plumbing!
each standpipe has a t section with a valve, when opened it will drain the tank to 50%. but obviously the water has to go somewhere...
so i have a t connection on the main pump line, and each hose has a valve. one line pumps water up to the top tank, the other one has a garden hose connection on it. normally i would have the main pump line open, and the hose one either closed or somewhat open (can control the amount of flow thru the system this way).
then to do a water change i connect a hose to this second line, and open that valve fully, then close the valve going up to the top tank and water is now being pumped out of the system. as the bottom tank drains i just have to open the standpipe valves and can basically just sit there while the water drains away. and because the water is being pumped out rather than using gravity, you dont get that damn 'bottom tank syndrome' where the lower tank drains at a snails pace and you fall asleep halfway thru the water change, or get distracted and come back some hours later to your fish flopping around in 1cm of water.
also, basically im sick to death of hauling buckets of water though the house!!
its a bit clumsy design wise this one, but at least i now know what i would have done differently
the next one will be four tiers high, all the matten filters will be on one side of the tank so i only need to access the one side, which will be better. also i will be making the drainage valves on the outside, at the moment they are inside the tanks which is kind of annoying having to take lids off and stick my arms into the tank to drain them. and will be using a larger diameter drainage hole, these ones have a bottleneck of 1cm and they dont drain super fast like i wanted them to.
oh and i will be using the normal straight style matten filter... its a bit too rigid to get a nice curve that sits flush like i wanted.
overall happy with the results, keen to set up the MKII tho... thanks must go out to jason for making the great stands, and ollie for the awesome tanks + plus general advice/helpfulness.