Ok you may not like this! But! You need to change more water more often. 9 in a 55 with one 50% weekly water change just won’t work. These fish need clean water. More so when under 4”. Even more so when the tank is overstocked.
Now just saying. The people who’ve really read up on these fish and want the best would be changing 50% daily. These fish will stress in bad water. Stressed fish get sick. Sick fish die. When not in the best conditions they don’t grow.
Foods ok. Being there eating granules allready look into the sera discus granules and simply discus discus chow. Both better foods.
Next I think your 3’ tank is a 50g. Here’s a pic of my 50g
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