Ich doesn't happen for "no reason". There is something that is stressing the fish to make them susceptible to illness. Be sure to get the the root cause of it to make sure you're preventing future outbreaks.
How is your water quality? Levels of Ammonia, Nitrite, Nitrate? pH? How often are you doing water changes to get the wastes and hormones out? How much water do you change at a time?
Which kinds/how many of each kind of plants do you have?
Do your old tank water parameters match the new water parameters at PWC time?
Are you heavily stocked or overstocked? (The answer is overstocked
, 137% according to AqAdvisor.)
Is there aggression in the tank causing stress? (The answer is yes
, see below.)
Do you have incompatibilites in temperature, pH, hardness, and activity levels? (Yes.
Agression- With even ONE Red Tailed Shark, the aggression will be high, much less TWO of them! Just one needs a 4 foot tank, and to be alone. This speciies is way too aggressive to be in this tank, and way WAY too aggressive to be with Discus! Your dwarf cichlids will also become aggressive when they start to breed, and don't have enough of a footprint to have enough territories. If your Discus or Scalare Angelfish pair, then that's a whole 'nother level of aggression on top of everything else.
AqAdvisor* sums up some of your problems pretty well: AqAdvisor - Timothy 1998 cremer
This tank does not have enough room for Angels OR Discus, much less both (and in the proper quantities)! The Angels are too much for the Discus, they can't live well together. The different species in your tank have different temperature needs that don't over lap (e.g less than 78 degress F for the Bolivian Ram, more than 80 degrees F for the Blue Ram). You have soft, acidic water fish AND hard, basic water fish. This tank is too short for Discus and Angels. Your shoal numbers are too small on Discus, Scalare Angelfish, Neon Tetra, possibly others but you have no room to expand their numbers.
There's more, but what I'm trying to illustrate here is that, if you don't resolve some of these issues, Ich will certainly not be the last of your problems. I hate to be the bearer of bad news but, for the welfare of your fish and saving you future strife, this tank needs an overhaul badly.
*Not that I think that AqAdvisor is the end-all, be-all... but it is certainly a helpful tool.