So this is a cycled tank (0 ammonia and nitrite and roughly 5 nitrate, filter, heater and 30 litres with live plants ) I’ve had no issues with it until about 6 weeks ago. I had 4 Guppies and a Betta in there, they all got along great. No aggressive or territorial behaviour from any of them. The guppy in the picture was purchased with some of his fin missing, that didn’t happen while I owned him.

As I said, no issues until about 6 weeks ago when I noticed one of the guppies seemed a little off, just swimming a little slower and resting a bit more. After a couple of days he died. No marks or indication he had been attacked, nothing to indicate any disease just seemed to die for no reason. I put it down to me buying the fish from PetBarn (equivalent of Petco in America) and thought nothing of it. 

A couple weeks later the exact same thing happened to another guppy. Swam a little slower one day and then died. Still just put it down to buying lower quality fish. I did a water test and parameters were as they are now. Did a water change anyway and all the other fish looked completely fine. 

Now last week the third guppy died. Starting to think there is an issue but just can’t figure it out. Parameters are good, none of the other fish seem affected, there aren’t any marks on the fish to indicate a virus, the dead fish don’t look attacked, no pine coming etc. 

Now today my Betta has died. Yesterday he was resting a lot and kept coming up to the surface for air. I was worried about an ammonia spike but there was 0.  I’m down to a single guppy now and I just don’t know what’s happened. 

The only thing I can think of is we have had horrifically hot weather lately , I’ve had to turn the heater off a few times. The temp is normally 24 degrees and I’ve turned the heater off when the temp got up to 27 degrees. The weather where I am is very strange so we had a couple of days where it was 40 degrees followed by a day where it was 24 degrees. So some crazy temperature swings but I also have 6 other tanks and no issues with any of the others. 

I want to restock the tank but I don’t want the fish to keep dying. 

What could be causing my fish to die?