I couldn't keep mosquitofish here in Phoenix in a small pond (30 gal) in summer. Too hot, too little O2, both. Generally in low O2 the bigger fish die first. That would be my first guess given the time of year and size of the tank. If the tank gets any direct sun at all I would like my guess even more. Above ground tank and even more likely.

I do believe mosquitofish will absorb eggs if conditions become unfavorable due to lack of food, temp, etc. At least it was seen with Gambusia holbrooki in a study, so I assume it's true for other kinds. It is also believed by many koi keepers that removing food for a month or so will stop females from spawning and cause eggs to reabsorb. So I think it may be a general fish thing.

I think goldfish do a good job of mosquito control when younger. I don't know about older bigger adults. But if conditions aren't good enough for mosquitofish they sure wouldn't be good enough for goldfish.

All the mosquito fish disappeared from one of my tanks, but the 3 goldfish seem happy enough. I assume they've been eating eggs and fry.

Thanks for the replies.

The tank is 45 gal. There are strategically arranged small trees, upright logs and potted plants on the sunny side. It sits under a big tree.

There is an old-fashioned corner filter running simply for circulation and aeration. In past summers, the water has not gotten very warm in the afternoons.

I kept looking for the mos fish babies which come each spring, but instead most of them died. The first to go were "pregnant" females. The water tests are fine.

I ramped up water exchanges and so far no more deaths.

Wish I knew.

Another question for you waterbugguy and anyone who would know: The comets are doing their springtime chasing. It seems a little late, but things are "off" this year. There is one which I know is a female who has a widening of her body behind the gills. She was always a stout girl, but this is a change. There is pineconing over that area only. Do they look like this with eggs, or do I have another problem? She is acting fine, eating, etc. They all got peas yesterday in case she is in need of a laxative. (!)

I'm confused if this is a pond or indoor tank.

Widening body behind the gills doesn't sound like eggs. That happens more mid body and low on the body. Pine coning is never a good sign and is not associated with eggs. However please keep in mind pine coning is only a symptom. Diagnosing the actual problem is difficult. The worst thing you can do is guess and start treating fish for every possible problem, especially for people new to treating fish. There's a reason want doctors and veterinarians have to goes thru more than 15 minutes of reading on the internet.

Peas do no harm, but very unlikely this is a digestive problem because of the location of the swelling.

The very best thing you can do is keep water quality as best you can. That means testing ammonia and KH and adjusting as needed. Water changing are good as long as you know what you're doing. Which means testing water.

Springtime chasing has more to do with temp and chemicals in the water than it does with time of year. So water changes can produce spawning activity.

Thanks! I have searched and can't decide what is wrong. Her swimming is good, appetite great. I don't think it's swim bladder dis. She got an epsom salts bath and did fine, but the next day reacted poorly to it, so I put her back right away.
Outside stock tank, shaded by trees, etc.