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Can anyone help me ascertain why this tank suddenly crashed on me? This is a 54 gallon corner aquarium, dirt capped with BDBS that has been set up and running a bit over three years. It is well planted with jungle valisneria, an amazon sword, a dozen or so java fern, a floating mass of hornwort that covers about half the tank that I pull handfuls of out every week during tank maintenence, and ever present duckweed I've never been able to get rid of. I net a fair bit of that out every week too. It has two sponge filters and a hydor canister filter, and an eight inch airstone bar, so I can't see lack of oxygen being an issue. Tanks stock was originally just six cardinal tetras, a clown pleco and some spare male guppies along with ramshorn and malaysian trumpet snails, and a single nerite. For the past year it has been stocked with 17 young angelfish that I have been growing out from dime size - they were about two inches in body height at the time of the crash, and had spawned several times in the tank, although the eggs had been eaten shortly thereafter each time. The angels were being grown out in the smaller tank with the plans to move them up to my 220 gallon once they reached a good size - I had been planning on moving them at the end of August. The angels, as they grew, ate all but one of the guppies, whom they could never quite catch. These were males that hadn't made the cut for best breeding stock anyhow, so I wasn't concerned that the angels had extra protein snacks. However they left the cardinals alone. Two days before last week's water change - I added three anubias which I had purchased online after giving them a hydrogen peroxide dip and then rinsing them. This is the only addition to this tank I have made in a year. I do weekly water changes of 40-50%, and test every week or other week at the time of water changes as well. Tank parameters at last week's water change were Ammonia 0 Nitrite 0 Nitrate 5 pH 7.8 Temp 82F TDS 345 and kH 5 (no gH test, haven't got one) These have been the normal parameters this tank has been testing at for well more than a year. Its been very stable with only the TDS ever really shifting. Last week two days after the water change, I noticed a dead cardinal tetra stuck in the hornwort and a spawn of angel eggs laid on one of the valisneria in the morning. I netted out the dead tetra, and tested the water parameters - Ammonia 0 Nitrite 0 Nitrate 0, pH 7.8 Temp 82 TDS 220 kH 5. I did a 20% water change anyhow. The eggs were eaten by that afternoon. Two days ago, I woke up to find three dead angels on the bottom of the tank. What on earth? Pulled the dead angels out and immediately began a 50% water change while testing the parameters. Ammonia .25ppm nitrite 0 nitrate 5 pH 7.8 Temp 82 TDS 300 and kH 5. Still the normal parameters. I'm not surprised by there being some ammonia with three dead angels but why did they die? The other angels and the cardinals were still acting normally, no red gills, no flashing, no sitting on the bottom nor gasping at the top. I left to go to work - I came back and there were four more dead angels and all but one of the tetras was dead. Pulled them out, lost another two angels and the last cardinal before the night's end. I sat and watched, they were going from swimming normally to sitting on the bottom and then stopped breathing within ten minutes. The next morning two more angels were dead. Itested the water parameters again as I did another 50% water change - still the same. In desperation I pulled the remaining angels out and threw two into a 47 column that only had some guppy fry, ottos and shrimp in it, and moved the other four to the 220 gallon - two of them passed away within the hour, I suspect system shock because the temp in that tank was only 78 and the pH in that tank due to large amounts of dirftwood is 6.8. But the other two made it. It has now been a day, and I have two remaining angels in each the 47 and the 220. I am waiting to see if they still suddenly drop dead in these other tanks. The clown pleco and the male guppy are still in the 54, but have not died. What am I missing? Could the anubias have brought something in that kills this quickly? What did I screw up so I don't have this happen again?