Toxins in the tank?

[Synirr]

I had a 29 gallon saltwater tank until a few months ago (it has been converted to freshwater,) but the first stock I had in it died of unusual causes, and I've just now decided to ask for some opinions about what might have happened. I cycled the tank with a couple of black mollies, and when I was sure the stage was set, I added a couple of yellowtail blue damsels, a mandarin goby, and some hermit crabs. Everything was fine for about three months, then one day I came home to a dead damsel... There were so physical signs of illness on the fish, so I checked the water parameters and they were all perfect except for slightly high nitrates (nowhere near a dangerous level,) but I did a 60% water change anyway. The next day, the other damsel was struggling to swim and eventually died. Water parameters were still ideal, but I did another water change, this time 70%. Two days later, the mandarin died after having the same problems swimming as the damsel did. The crabs survived, though.

Now then, my theory is that some sort of toxin got into the water... something aquarium test kits don't normally test for... All this started happening the day after our cleaning lady came (she comes once every two weeks,) so I was thinking that maybe she decided to clean the outside of the tank with glass cleaner and accidentally got some inside? Would that have done it?
I wish I had thought to take a water sample to my LFS, since they test for more than just the typical toxins...

 

[Navarre]

A highly likely cause Any form of air freshener, polish or the like is a danger to marine tanks.

You didnt mention the type of filter you had, it might also be a filtration problem. If their was an ammonia spike it might have happened so fast that when you got back home the spike had gone and left only nitrates. There is also the temerature to think of. The problem with a small tank is that the water can change so fast. The temperature may he risen high enough for the fish to suffocate and by the time you got home the temperature was again normal. Remember that salt water fish need a oxygen rich water, far more than most freshwater fish do.
Hope this helps.

 

[Synirr]

A highly likely cause Any form of air freshener, polish or the like is a danger to marine tanks.

You didnt mention the type of filter you had, it might also be a filtration problem. If their was an ammonia spike it might have happened so fast that when you got back home the spike had gone and left only nitrates. There is also the temerature to think of. The problem with a small tank is that the water can change so fast. The temperature may he risen high enough for the fish to suffocate and by the time you got home the temperature was again normal. Remember that salt water fish need a oxygen rich water, far more than most freshwater fish do.
Hope this helps.

Thanks for the input... I forgot what kind of filter it had, but it was more filtration than required for a tank that size, for sure. Our house has central air and heat, so a temperature change is damn near impossible, and the tank had plenty of air stones to supply oxygen... That's why I figured some sort of toxin was the only likely cause. I set the tank up as saltwater once again after a thorough cleansing and it had no problems after that.