Everything Dead in FOWLR

Question
QUESTION: Hi there Taylor.
I have a 20 gallon FOWLR tank that had a pair of perculas, a tiny valentini puffer, a green spotted mandarinfish, and 2 hermit crabs.
I did a 25% water change the night before and fed the fish the next morning, with all of them swimming normal and feeding as usual. I came home after work (approximately 5 hours after the feeding) and found the female percula, puffer and mandarinfish all dead. The male percula is breathing heavily and the tail half of his body is covered in a white fungus/fluff. What is wrong and what can we do to possibly save the remaining fish.
The hermit crabs are still alive and we have noticed that a large number of copepods and anphipods have also died and are lying on the substrate.
Please help asap!

ANSWER: Hi Arylene. Sorry it took so long to get back to you and I wish i would have been home to help you sooner. There are many things that can cause what is known as "toxic tank syndrome". Usually if all your fish were doing fine previous to doing a partial water change the first place to look is to your water supply that you used to do that partial. If you are using tap water weather it be city water or well water there can be small amounts of different toxins that can come in with the water that will not effect you but even in small quantities are highly toxic to fish. I would surely suspect this to have been the problem considering that you found a fungus like growth on the only remaining live fish. When a toxin enters your tank the first sign will be the effect it has on the fishes skin and breathing. If you use city water most municipalities will use different anti-bacterial chemicals during different times of the year to help combat these bacteria from entering the mains water supply. If you use well water there are alot of things that can enter a well, especially after a heavy rainfall, due to run off. If on the other hand you used bottled distilled or reverse osmosis water(which is recommended) then you may want to look at where the equipment that you do your partials is kept and what could have possibly come in contact with it. Some other common toxins that can enter your tank are any aerosol sprays, room deodorizers, pesticides, even the fumes form Teflon cookware can enter your tank at the waters surface and be toxic to your fish. Heavy metals, rancid fish food or a power outage can all be suspects as well. The list can go on and on. Sometimes you may never know what exactly caused the problem. Use of fresh activated carbon and very frequent partial water changes are a good way to start to get everything back to normal. I am so sorry to hear of your loss. Just remember that it was most likely nothing you did wrong and it is one of those things that can sometimes happen even to the most advanced fish keepers. Don't be afraid to start over and try again. It will take some time and patience to get your tank back to normal. Sorry I didn't get back to you sooner!

---------- FOLLOW-UP ----------

QUESTION: Thanks for your reply!
I managed to call someone from my LFS and they advised me to set up a QT for my little survivor and medicate with API Pimafix. He seems to be doing well so far, 2 days into quarantine, and the white fungus has subsided significantly. Unfortuntely, he's still got skin lesions where the fungus was and his breathing is still very laboured. I hope he will get better eventually.

On another note, what do I do with my main tank? Do I have to re-cycle the whole system and replace the live rock in the tank? I really don't want to do that but I'd imagine if it were toxins that caused the fish stress, that my tank is pretty much dead. Will water changes and activated carbon be enough to save it?

Answer
Hi I am glad to hear that one little guy made it! If there is nothing left alive in your tank then you should just drain it and start over. Using fresh activated carbon in your filter will filter out any toxins that might have entered the tank. I don't think you have to worry about changing all the rocks or anything just give them all a good rinse. Same goes for the filter. You most likely do have to cycle your tank all over again. Whatever caused everything else to die most likely killed off your good bacteria anyway so don't be too concerned in saving them. Do a really good cleaning changing all the water and rinsing out everything else. Use reverse osmosis or distilled water to mix with your salt when refilling and let it all filter through fresh activated carbon for a few days before adding any living creature. When you feel ready add just one of something(maybe a molly since they can live in saltwater and if it dies its no big deal since they reproduce like crazy)then see how it does. Don't get discouraged! Your tank will be up and running and as good as new in no time!