Discus hiding

1. Please explain the problems with your fish. When did you notice the problems and did anything unusual happen that you think started them?
I performed a 30% water change yesterday and today one of my discus is hiding at the back and not eating. Issue is 2 weeks ago one of my discus died, I put it down to "bad genes" in the fish, as he never put on any size in the year I had him despite treating him for various things and he seemed happy enough in the tank (he was eating and swimming actively with the others until his last couple of days).

2. Symptoms (i.e. turning dark, excess slime, not eating, clamped fins, flashing, darting, clamped gills, white/yellow/green poop, hiding, headstanding or tailstanding, white on tips of fins, rotting or fungus, blisters/white zits on fish, bloated, cloudy eyes, wounds).
-not eating

3. What medications/ treatments have you already tried and what were the results. Include dosage and duration of treatment.
-None - Believe it maybe too soon to start panicing - just wanted some more experienced opinions

Tank/Water

4. Tank size and ages, numbers and sizes of fish.
- Tank size 60 gallons 2yrs old
- 2 adult discus, 6 clown loaches, red tailed back shark, Neon cardinals, Golden nugget Pleco and Queen Arabesque Pleco

5. Water change regime (What percentage and how often).
I perform a 30-40% WC every week and have done since set up

6. How long has tank been running? Is it bare bottom? If you have substrate, what type and how deep is it?
2years
It has sand substrate(half inch depth)

7. Do you age your water? If you do for how long and what is the ph swing.
-No

8. Parameters and water source;

Note: Water Parameters are important in diagnosing problems within a tank. If you don't own test kits for the following information, you can purchase them, test your parameters and post this info as soon as possible.

- temp 28 degrees

- ph 7

- ammonia reading __0__

- nitrite reading __0__

- nitrate reading __20__

What type of water or combinations of water sources do you use? If it is an RO/tap/well water mix, please list percentages in the mix.

- RO water

9. Any new fish, plants or inverts added recently.
-NONE

10. Please tell us what you feed your fish and how often. This can be critical information for solving the problem so be as specific as you can.
fed once a day - each day is different... Blood worms, Mosquito Lavea, Discus basic food, Shrimp, Krill, flakes

11. Include any pictures or videos you have which shows the symptoms. If you can't add them to this post, please provide a link to them.
https://www.dropbox.com/s/1y3viu9mkl...41.27.jpg?dl=0
https://www.dropbox.com/s/l1m9erf6qp...41.17.jpg?dl=0