Hello; I'm not sure if I'm just an idiot or missing something but I have an ongoing issue. I went on vacation and left the care of my fish to my nonfish loving husband and while gone the Goldfish pond outside went toxic on nitrites. Fish were jumping out and being eaten by my dogs and by the time I came back I only had six ragged Sarasa comets left. I put them in a new tank with a filter from a cycled tank plus a cycled sponge filter in the house. Every day the nitrites would build. I made the mistake of moving them to the Angelfish tank because it was bigger at forty gallons. This move eventually killed those eight Angelfish and five of the Goldfish died. I moved the one remaing Goldfish back to the other tank. I did a complete water change and in one week I added ten new Angelfish and within two weeks they succumbed to such things as fin rot, swirling sideways and one by one they all died. The question is: I did water changes every other day from the moment I put those sick Goldfish in that tank, water parameters were at 0 for Nitrites and Ammonia and about 10-15 on nitrates. The question is: I am wondering if there are bacteria in that tank from the nitrite poisoned Goldfish that are hanging on and if so must I tear down this tank and kill all with bleach? Thank you for your help and sorry this is so long winded. Debbie