I was very lucky and managed to buy 36 40ltr tank the same size of TradeMe for only $300! All the tanks were pre-drilled at the bottom and came with all the fittings and drainpipes - best deal of my life !!!
Now that I had the measurements of the tanks I could start maximising the space and building the racks. I used MS Excel to calculate all the measurements for each level of tanks and the total hight. I started from the bottom and since I wanted to siphon the tanks into the drainage system and wanted to have a good view of the bottom layer while sitting in a chair, 45cm was the goal. From here on I worked my way up to the top tank which is 1.6m above the floor - perfect hight while standing.
Now that the measurements for each level was sorted I started building the racks. For the uprights I used 90mm x 45mm wood. In that I cut out 70mm x 45mm to slot the horizontals in. That way the weight is not just been hold by screws. Since the total length of the big rack is 2.10m I needed a support beam in the middle. The rack on the left side is only 1.20m long and doesn't need a support.
The racks are sitting on beams which distribute the weight to all the floor joists and not just on the little area of the racking feet. Since the racks are only 55cm deep they are secured to the walls.