Got 2 figure 8 puffers (about 2-2.5") in a 20L about 1 month ago (Nov 29). Aside from the larger puffer not eating at first, they have been doing fine. Tank was cycled, pH 7.8-8.0, brackish water. Last water change on Sunday 12/23 - 50%, cleaned the decor due to a brown algae bloom (which I don't understand in itself because nitrates have never been over 10 since the puffers were put in it -- maybe too much light...), added another small HOB filter, and some more plastic plants (well rinsed). This morning, the larger of the two puffers did not look his normal self and did not eat, went to Christmas dinner at friends, and when we got back tonight, he is doing very poorly and appears to be on his last fins. His skin has tiny white/yellow spots kinda where you'd expect spines (?) to pop out (very patterned and evenly spaced) and looks kinda pocced. His belly is extremely concave but he looks slightly enlarged around his anal fin. He is sitting on the bottom, hardly breathing, with an occassional burst of swimming only to wind up at the bottom again. Tested the water, pH is 7.8, Ammonia 0, Nitrites 0.25, Nitrates 5.0 (did I kill off my bacteria during the last water change? Prior to water change, ammonia and nitrites were 0, nitrates 5). However, if it was due to the tank parameters, wouldn't both fish be suffering? Could he have parasites? By the time I get a response, I think he will be dead, but I want to keep the other one alive. He appears to be doing fine, eating and swimming around like normal. Any suggestions on what might have happened with the big guy? What does one do when they know the fish is dying, but hasn't gone yet? It pains me to watch him struggle...