Over filtration is not the answer, maintaining stable and appropriate water conditions is. A 12" deep tank is not really suitable, 48"x18"x15" is a minimum and bigger is better if you can. One external filter of an appropriate size is all the filtration you require, you can over filter but IMO its really not needed. Over filtering cannot replace frequent water changes no matter how many filters you run.
Buying large discus is very good advice, and as your a beginner you should avoid a heavily planted tank. A bare bottom setup would be best, you can add plants in pots or on bogwood if you wish. Small discus are easily stunted and should be avoided for beginners IMO. The discus you buy should be pretty much perfectly round, have good colour and size around 4". However, you need to be prepared to spend the money on decent quality fish, such a discus will probably be around £40-50. If money is tight this fish isn't for you.... but hopefully you already realise that. You can keep 5 in the 48" tank mentioned above, but keep tank mates to a minimum. Water should be kept at 30C so you'll need a decent heater, I run two in my tank, also be prepared for the higher temp to add slightly to your electricity bill.
You'll need to keep them in soft water, and unless you are very luck to have very soft water from the tap the easiest way to soften your water is by using an RO filter. More info on this is available by searching the forums.
Do a bit more research and if your still interested then asked any specific questions you have.