I have had a 55 gallon planted discus tank up for about 4-5 weeks. I noticed a couple days ago that one was darker than normal. I thought it was stress, did a 25% wc tested,the water, (ph-7.6; ammon-0; ites-0; ates- 0). His coloration is still dark. I have not seen him eat, but he is not.losimg weight. The temp is zround 84. Then this morning i noticed another is dark and he looks like he has a whitish strand of dust hanging from his gill. The only thing i changed was addind excel to the tank for the plants. And i have noticed brown algea on some of the plants since then as well. I have not added anymore thinking this may be the problem. Any thoughts are appreciated.