i need some tips on discus fish?

[fish boy]

Hi, ive been looking ater some mbuna cichlids for just over a year and ive been doing really well with them but i feel like a change and my cichlids are leaving 2morrow. I've decided to try the discus cichlids, i know their difficult so ive been reading alot of websites about them and have asked my uncle who use to keep some so i have a good idea about what they need to thrive but i still have couple of questions
1. my tank is a 30 gallon so how many would be best
2.how can i make the water soft and keep it like that when doing water changes, same with keeping the PH down
3. i have a under water filter which is a fluval make, will that work or do i have to buy i speical make.
4. i have 2 pleco's will they harass the discus

Only give advice which relate to my questions nothing else unless its a tip which not many people know about

I have tested my tap waters ph, ammonia and nitrite the results were...
PH=6
ammonia=0.10
nitrite=0.00

thanks to anyone who answer, ok bye
ive heard that each fully grown discus needs 10 gallons each is that true or is it more
i think i made a mistake when i said the filter was an under water , its not actually under the gravel also i am going to start with a few small 1s around 3-4 inches.

All the websites ive looked at say that 30 gallon can just about work with discus