So you going to do a 125 Gallon Cichlid Tank. That's awesome and allot of fun 4 you.
What type of Cichlid's ? African, South American, ? etc.
Get a good solid tank. And matching hood and lights.
Make sure you get a matching stand that will hold the weight of the tank.
Filtration is important in a Cichlid tank. You going to want something that will filter for about a 200 gallon tank. Maybe a Fluval fx5
As you know Cichlid's produce allot of waste.
I have a 55 gallon with a fluval 405 and to Marineland 280's.
Substrate is important. I like the sand substrate
because of ease of maintenance. This stuff comes in black and white. Also will maintain the PH that cichlid's need. So here you do not have to worry about keep the proper PH.
Decor: Cichlid's love to hide. Rocks like limestone would be a good addition
for making tunnels, caves etc. Plastic plants are best when having Cichlid's.
As you know they like to landscape.
I put in a center piece that looks like coral and it has lots of tunnels and built in air stone. You need to study the natural habitat and decide how you want it to look.
Heater: Probably a 250 to 300 watt Marineland I think is the best JMO.
Air Pimp: Whisper or any that you choose.
Then of course you will need the maintenance tools to keep it clean: Gravel Vac, Magnetic glass cleaner, etc
Chemicals: Water conditioner, I use natural trace elements for cichlid's,
There are now many product's made just for cichlid's. Cichlid rift lake salt really makes their colors pop. But not to over do it.
Just some suggestions, sounds like fun.