I set the tank on the m/o the other day to make room for other stuff.
Today they are all dead.
Did I doom my fish?
Did you cook anything in the microwave while the tank was there? If not, then it shouldn’t have had any effect on them. If you did and any part of the microwave shielding was defective, even a tiny little crack in the right place, then yes you might have parboiled your fish. Even if the shielding was OK, my microwave has a vent in the back where heated air inside is vented out, and that might have heated the water in the tank enough to kill them.
The shielding is designed to keep radiation levels below human thresholds. Fish are different. If you used it while the tank was on top, I’d convict you of murder.
I seriously doubt that any microwave radiation can leak out of the top of a microwave oven - it’s a solid sheet of metal. But, it might get warm, which could be enough to kill the fish.
The “radiation” in a microwave is just radio waves (specifically radio waves in the microwave frequency band, around 2.4 GHz). While it is technically electromagnetic radiation (as are all radio waves, and light waves, etc) it’s not “radiation” in the sense that we typically use the word (ionizing radiation, like x-rays and gamma rays).
The radio waves in a microwave are generated by a thing called the magnetron. When the magnetron isn’t energized and spinning, it isn’t generating any radio waves at all. There’s no residual “radiation” of any sort hanging around a microwave. If you turn the microwave on, there will be rather high power levels of radio waves inside the box, which is how it cooks the food. Radio waves bounce off of metal, so they pretty much stay inside the microwave box. The amount of radio waves that escape is pretty small and isn’t worth worrying about. Unless you put your guppies inside the microwave and turned it on, the microwave couldn’t have caused them any harm from the radio waves.
Microwaves are fairly inefficient and generate a lot of waste heat, which eventually ends up heating the air around the microwave. As was already mentioned, this could heat up your fish bowl a bit. I don’t know how sensitive guppies are to heat. I can’t imagine that they would be so sensitive that placing them near a microwave would cause them a problem. If they were that sensitive then placing them near a furnace vent would be just as deadly.
Is the microwave near a window? Is there a possibility that sunlight heated the fish bowl? I could picture that causing enough of a significant temperature rise to kill fish.
I can’t imagine even a damaged microwave leaking enough radio waves to kill fish, unless the thing was so badly ripped open that it would be pretty obvious that it had a problem. Keep in mind also that the fish are in a bowl full of water. The radio waves would have to heat the water as well as the fish.
Ionizing radiation like x-rays and gamma rays can be deadly like this, but radio waves are too low in frequency to be harmful in the same way.
Almost everything said by others about about radio waves is correct. But you said you set the fish bowl on top of the microwave. If you used the microwave, then the heat probably did kill them. The top of a microwave can become nearly too hot to touch, certainly uncomfortable. That heat would directly conduct to the water in the bowl and kill fish in minutes. I would guess that if you used the device for more than three minutes it could be a factor. Heating a doughnut for fifteen seconds would not. You be the judge of yourself.
How long did you have the guppies? How big was the tank and how many fish did you have in it?
If you just recently set up the tank, or if you had a lot of fish in a small tank, then I’d suspect that the fish died of “new tank syndrome”: New Tank Syndrome
The reason I would suspect this is that water quality problems are the biggest killer of pet fish, especially in small tanks or fish bowls.
I’ve encountered microwaves that “leaked” enough to noticeably warm my hand from ~6 inches away. Usually that was somewhere around the door, however, and I’d be surprised if any microwaves “leaked” out of the top. But there is plenty of ordinary waste heat on the top of a microwave, as others have mentioned.
I’d be surprised if it could heat up that much- glass is not a very effective conductor of heat, (or acrylic, if the tank was made from that) and unless you were using it for a very long time, guppies would probably be OK. They’re not massively sensitive to temperature changes in my experience.
If you’re keeping them in a tank small enough to carry round and put on top of a microwave though, that really sounds too small- they are very sensitive to poor water quality. I gave up keeping them, as, due to overbreeding, they were very prone to problems, even when my other fish were fine.
I have had several microwaves in my life, and never did the top get the slightest bit warm when it was turned on. Maybe you’re thinking about a toaster oven – some cheap models have no insulation between the heating elements and the metal top, and it can indeed get burning hot.