I would say a about 20lr per fish but its the shape of the tank thats important you want depth over anything els now you got to decide on how to keep them their are two schools of thought and alot of mad idiots in the midle the first are the import fresh from the amazon with cleen bottom tanks with sterile RO
water 50% water changes every other day
I would never do it this way it is cruel by my way of thinking
The other way is with what is known as a blackwater tank and you use indian almond leaf and bark and a few other bits to add the same stuff that they swim in in the wild just so happens to be an antiseptic this is why you cant just use tap it seems to me to be obvious after my original research but its the breeders that first got it but the information never got back to the home tank market
You will whant to plant the tank with grasses and other discus plants good mixes avaliable on ebay and I would recommend wood and bang it in straight away
Set it up with lots of good spots to hide now add 20-50 cardenals or neons I like to have 10-30 cores a few dwarf rams ether the yellow or the blue now it is your choice you can have any of the calm tetaras not blue or congo but dimond rummenose phantom widow are all good oh and no daeos and go get good test kits your going to need alot
Now this is how I start up a tank fill tank
Put in classic undergravel filter then add washed gravel add wood and plant use standard airlift on under gravel filter and add an external filter with around 10 to 12 times the volume of a tank in an hour can use a trickle bar return so as to not have too much current
Now you need a dirty filter but it needs to be fresh within 4 hours of removal from the other tank wash the filter in your tank this sounds insane but it will kick start your filter better than any bottles rubbish you can get in your shop
Now wate a week testing every day......
If you have your tank stable time to get some fish but not your discus
Now you have to do the boring bits and wait for around 6 months walst levels stabilize in the tank and you have to get to a month with no disease at all you can do this faster but you will most likely loose fish and it wont be the tetras that go first!
Do this read more books but only new ones the old ones will kill your fish with bad stupid and just plain wrong advice
All in all good luck you will need it its an expensive love affair but when it all works it is amazing I can watch my tank for 2-3hrs at a time