Last year, my wife bought 3 red Mollies for our tank. They shared the tank with some Fancy Guppies. Within a few days of being in the tank, one of the mollies had disappeared. There were no bones, nothing in the tank. Then, a couple days later, another one disappeared without a trace. There is a lid on the tank with only very small openings around the filter housing. I seriously doubt they could have jumped out.
So, for a long time now, we have had the tank with Guppies and the single remaining Red Molly(If it makes a difference, he is the biggest Molly from the group). This week, we added 3 new Red mollies to the mix. Guess what? Within 3 days, 1 was already missing!!! They don't seem to fight, and i have a hard time believing the Guppies swallowing him up.
This is so embarrasing. My wife just realized that the fish are red-wag platties, not mollies. Yes, we notice them missing within 24 hours, I'm sure of it. We spend a lot of time watching the fish. Us and the 4 kids!
Yesterday afternoon, a guppy died and I left him in there as a test. He is still there this morning, nothing ate him. And he's 1/4 the size of the platties. So if the platties were being eaten, why not the guppies who die?
I'm not an expert, but I wouldn't rule out the guppies. Mine were absolute carnivores, although they would eat anything. Platy are supposed to be omnivorous, and mine are, but they are the first to attack a slice of zucchini and will have it eaten in 24 hours, or less, and I only have three Platy.
I've noticed that some fish seem to disintegrate faster than others. If your filter intake has a large opening it's possible that the Platy were pulled apart and sucked into it and stayed in your filter until completely gone.
Also, when one of my Platy dies, it's body becomes translucent and barely visible, and this happens fast. I've noticed that the guppies seem to lose their color slower.
I agree with Zambize with the intake tube. Mine would get trapped against it after the lights go out and everyone is resting along the substrate or just hanging out in one spot and get sucked to the tube. I turned the light on real early in the morning and seen half of a platy (the head and gills) stuck against the intake tube and the other fish had eaten the other half of the body before I got to him.
We have a small filter (the tank is only 10 gallons) and it's really not that strong. Substrate would be small pebbles. Could the guppies actually overpower and eat the platy? Or maybe they just die and are eaten overnight? Could I test this, maybe by dropping a 1/4 lb of hamburger meat in the tank to see how long it takes for the guppies to eat it?
I can't see a guppy eating a mollie or platy. They are bigger than a guppy, but they will pick on them or try to mate. My male guppies will chase a female platy or mollie around if there is no female guppy in there, but I don't think they can do anything with a platy. My guppies only eat fish flakes, but the platies will eat $#@ if you put it in there. lol
Also, my 10 gallon tank with little filter sucked up 2 betas and have seen what was left of a couple or so of platies that was attached to the pipe overnight.
Have you checked all through your substrate for any signs of what used to be a fish? A few months ago, one of my gold nugget plecos died and when I was vacuuming the gravel and moved the tree trunk, there was a little skeleton that resembled the pleco buried under the gravel.
We haven't really dug too deep this time. Last time, we did move things around a bit, thinking maybe the Platties had somehow burrowed down into it, but found no trace.
