First thing is what kind of tetras are you talking about? Secondly, do you have experience with discus? Thirdly, discus don't live in the same water conditions as the rest of those fish, they need warm water. I personally would out the discuss and stick w/ rams. Discus are a lot of work especially if you don't particularly know what your doing. And yes get ottos. Personally I would throw in 18 or 20 rather than 3. They're too small to really be noticed unless in large numbers and they don't really make a dent in the algae unless you have a bunch. I have 16 in a 60 gallon... no algae here. Otherwise the setup looks good.