This is a subject that facinates me. How people organise the space in the cabinet below the tank.
Here's my contribution. The cabinet is organised using cable management trays, to keep all the wiring out of the way. I fitted all the power cables with Neutik PowerCon True 1 in-line connectors, to facilitate removal of the equipment without having to pull all the cables out of the cable management trays. You simply pull back a slide & twist and the equipment can be removed, (these connectors are popular with musicians, for all their equipment).
I made up a temperature monitoring/control unit using an STC1000, and based the design on several on youtube and various forums. It is capable of controlling both a heater and a chiller, keeping the temperature within +/- 1C. I simply set the heater's control to 1 degree above what I want, and then if the heater goes rogue, no harm, as the STC shuts it down. The same applies to a chiller. If the STC faults, it shuts down and sounds an alarm, (popular with the home brew crowd).
Water comes down from the tank & directly into the Fluval 406, (centre), then outputs to a Hydor Eth200 in-line heater, and then a Sera CO2 reactor 1000, which I have plumbed in, (along the right hand wall), in clear acrylic and vinyl tubing. (the whole assembly is held in place using marine plastic clips, and is easily removed for cleaning/servicing/replacement of equipment). The Fluval's quick release system allows easy removal for cleaning/servicing.
On the left side is the CO2 cyclinder, which feeds the Sera Co2 reactor, via CO2 proof tubing, which runs across the inside top of the cabinet, and held in place with some 3M clear picture hooks. (the small ones are just right to hold 6mm CO2/airline tubing). I solved the problem of 'air gurgling' in the reactor, by fitting some in line valves in the CO2 tubing and a t-piece, that enables me to 'bleed' the air out, when it first starts up. A one-way check valve is fitted in-line on the cylinder side of the 'air bleed' set up.
The second picture shows a timber 'cube' unit I have set up to hold most of the other things I need regularly, for the tank. I have fitted acrylic shelves (custom made) to the side to hold food, fertilisers, water conditioners, bacteria cultures and the tubing I use for water changes. The main shelves hold test kits, more food & water conditioners, and general cleaning items.
I hope you enjoy, and maybe it gives you some ideas for your set up.