Sudden guppy deaths!?

[MOsborne05]

My brother recently upgraded from a 10 gallon tank to a 20 gallon tall. He started out with a pair of guppys which had babies. Only two babies survived and they were both males, then the female disappeared which left him with 3 males. After the tank had been running for a couple of weeks he added 3 female guppies. Two of the males were still pretty young so we didn't think 3:3 would be a problem and he was going to get a few more females eventually. They got along great and started swimming together right away. They have been in the tank for about two weeks now, and yesterday all 6 guppies were dead and stuck to the filter intake. Any idea what could have happened? Could the 3 new females have brought in a disease that just now killed them? He said that they all looked fine, there were no visible signs of injury/illness. I have no idea what happened but want to figure it out before he gets any more fish.

BTW, the tank has a pair of dwarf gourami's, a pair of bettas (yes male and female and there has never been any problems) and 2 otos. He got two tiger barbs last Friday and I told him to watch them closely since they can be fin nippers but so far they just chase each other around. I did tell him though, to either take them back or get a couple more. The tiger barbs were the last fish added to the tank, so maybe they brought in a disease or something? All of the other fish are fine though.

 

[FroggyFox]

Chances are good that a new fish brought something in. If it were the fish picking on each other and killing them all I'd think there would be some pretty obvious signs of trauma to the dead fish. The 20G with those fish in it sounds like trouble to me though...a pair of gouramis with a PAIR of bettas? They'd pretty much need the whole tank to be able to get away, so maybe one of the pairs was feeling too crowded. At any rate tiger barbs would probably be the LAST fish I'd add to that tank.

I think you can rule out poisons etc since no other fish besides the guppies died...but I'd caution him not to get any new fish for a couple of weeks just to make sure its not something he can't see that made them ill.

Also, you didn't mention if they tested the water for ammonia/nitrites...guppies could just be a lot more sensitive than any of the others to high levels of one of them because unlike the bettas and the gouramis they can't breathe air from the surface and the addition of more fish could have just been the last straw so to speak?

 

[MOsborne05]

He doesn't have a test kit and I forgot to take mine with me. I know its not a very big tank but there are lots of plants and hiding spots. The gourami pair sticks together, and the betta pair stick together and they leave the others alone. I know, I didn't believe it at first until I sat and watched them for an hour! Everytime I go over there I sit and watch them and I've never seen any aggression from any of them. I think the bettas are spawning right now too, there is a big bubble nest around the floating baby grass and the bettas have been living in the grass for a few days now.

Now that I think about it, it probably was either a disease that the guppies were more susceptible to or high levels that the guppies couldn't handle.

 

[saphirehaze]

Fish disease

I read about this disease in one of my aquarium mags that is carried by asian imported fish, but, for some reason, only kills angels and guppies. It is a virus. Also, its a good point about the gourami's and bettas not being affected. These fish can breath air to some degree, and cory cats also can use air as an oxygen source. At night, in a planted tank, the oxygen levels can plummet. If for some reason your dissolved O2 levels were low, guppies would be particularly sensitive. Its a mystery, but death of only one species in a community tank all at the same time implies a single, specific cause.