Was thinking the past few weeks about this and this is dangerous Will try and explain this best I can. So say you got a 200l tank that you want to do a 50% waterchange every 24 hours. Firstly will this be a good idea and benefit the system. Now the plan. 200l tank Then ad 2 x 200l holding containers The first container you fill with normal tap water. Then ad prime to declore. Ad KH up if needed and say equilibrium for GH. Ad ferts to desired levels whanted and so on. Basically get the water like you would want it going into your aquarium. When this is all done and the water was standing for say hours you transfer this water to the second holding tank. Then fill up the first holding tank again and prepare this water like you did with the first lot. Now you have 96 hours (4 days) of waterchange water. The idea for the two holding tanks and not one is that u always start with a known volume of water while always having some water for waterchanges. So when the first holding tank is completely empty then mix up another magic mix. If you know the parameters of your tap water you can work out how much of what you must ad to get the desirable parameters. No more testing or not that often just now and then just to see if everything is still in check. The waterchange can be done with a pump normally used for sumps with a tap to control the flow that you add in this case 100l per 24 hours. Then the aquarium will have to have a overflow going outside to the garden drain or wherever you want it but not on the lounge floor. My thinking behind this. Your water parameters should stay very stable. No pH shock with tap water added directly to tank. Seeing tap water normally has a higher pH when coming out the tap. No temperature shock because water added over a period of 24 hours or even better you can add a heater in the second holding tank if you want. Fertilizer quantity should stabilize to a set amount and never build up. As plant mass increase the balancing point should slowly adjust itself. Waste produced by fish and plants should be eliminated. This is just for waterchanges and you still add say a canister filter for filtration. Is this a viable option or am I really going crazy and need to go seek help.