Hi. I have a 72 gallon bowfront tank that has been up and running for about 3 months. It was seeded with 2 hydro sponges (both heavily seeded), has a eheim 2217 canister filter, about a 3 inch PFS substrate, and limestock and slate rocks for caves. I have 8 demasonis, 4 yellow labs, a syndontis multipunctatis catfish. I just bought 2 bristle nose plecos 3 days ago and added to the tank. Water parameters are ammonia-0.15, pH- 8.3, nitrite- 0, nitrate- 2.5. I run 78 degrees in the tank. I have 2 demasonis and 1 yellow lab holding at the present time. I feed NLS 1mm sinking pellets in the mornings and only spiralina flakes at night, 6 days a week, fasting them on Sundays. I put the bristlenoses in the DT 3 days ago (I buy all my fish from one guy and had never had to quarantine before, so I didn't this time). The day after putting them in the tank, we noticed the biggest pleco had yellowish-gold dust on her, with bruising type areas on her body. She was very lethargic and woudn't eat. We removed her and put her in a QT, treated with Mardel Coppersafe according to directions, woke up this morning and she had passed on. In the meantime, we noticed the same type dust on the other pleco that is still the main tank along with the cichlids and catfish. Some of the cichlids are scratching on the rocks and sands more frequently than they usually do. They are still eating, but unsure if the pleco is eating or is or not lethargic. He just stays in on one rock. The pleco in the tank has not started showing any signs of the bruising areas that i can see. Could really use info quickly to know if I should treat the whole tank with Mardel Coppersafe. If I do, will it kill all my beneficial bacteria or hurt my cichlids, catfish or the bristlenose pleco. Would like to get treatment started ASAP so I don't loose my fish! Please help!