Fish Disappearing In My Tank

So I have a 20 gallon tank and ever since I got it I’ve had random fish disappear.
I have had some fish die in it and I find either a skeleton or decomposing fish but a lot of them just disappear without a trace.
Today I went to feed my fish and I noticed my largest fish, a female Molly, did not come up to get food like she normally does. I looked around the tank and couldn’t see her and also asked another person to look and they couldn’t find her either. I don’t have many decorations in the tank and those I have do not have tight places where a fish could get caught. I have one plant where smaller fish have gotten caught before and I’ve looked through that plant but she is not in it.
I’ve had two aquatic frogs, a loach, a tetra, and two platys, disappear in my tank over the three years I’ve had it.
Right now there are only a guppy and a tetra in the tank as I haven’t been able to get more fish recently. Neither of them have even been aggressive and while they are both omnivores if they had eaten the Molly there would have been a skeleton left.
The filter for the tank is completely enclosed with no way of a fish getting caught in it and I have already checked around it for my Molly but she’s not there.

Any idea what’s going on?

 

Did you look around the floor behind the tank? Do you have a cat? Do you have a lid covering the whole top? I have had fish jump out and caught my cat on top of the tank fishing

 

Did you look around the floor behind the tank? Do you have a cat? Do you have a lid covering the whole top? I have had fish jump out and caught my cat on top of the tank fishing

There’s a lid covering the whole top and I don’t have a cat

 

Do you have live plants?

 

Do you have live plants?

No I do not

 

What type of filter do you have running?

 

What type of filter do you have running?

A fluval U series

 

I haven't had that particular filter but it may be worth checking the inside of it just in case.

 

Do you have snails or shrimp in your tank? I've had snails eat an entire carcass, bones and all one time. Also, how long has your tank been set up? Has it been through the nitrogen cycle? Just curious as to why the fish are dying off. Have you tested your water parameters?

 

Do you have snails or shrimp in your tank? I've had snails eat an entire carcass, bones and all one time. Also, how long has your tank been set up? Has it been through the nitrogen cycle? Just curious as to why the fish are dying off. Have you tested your water parameters?

Don’t have any snails or shrimp, tanks been set up for about two years so the nitrogen cycle is definitely through, I haven’t tested the water recently I’ll need to go get a test but the other fish in there are okay. Occasionally one just seems to disappear

 

No I do not

That almost certainly rules out dragonfly or damselfly nymphs then.

Don’t have any snails or shrimp, tanks been set up for about two years so the nitrogen cycle is definitely through, I haven’t tested the water recently I’ll need to go get a test but the other fish in there are okay. Occasionally one just seems to disappear

Maybe they're just getting old? Guppies and mollies will rarely live for more than 2 years (my oldest pure guppy lived to only 14 months, and it was old age as she visibly looked very old by then, others haven't even made it to a year) and that's also about the life expectancy of captive small tetras. Most of my old cardinals lived to about 2 years of age. Some rummy noses seem to live a bit longer, I have three 30 month old ones right now. I haven't kept glowlights long enough for them to reach life expectancy yet. Neons will be in the same boat as cardinals. Cyprinids of this category seem to live the longest; my last glowlight rasbora out of a batch of 8 I believe is just about 4 years old.

I've had fish seemingly disappear before. Both of my remaining old cardinals some months ago, two of the rummy noses in a similar time frame and some cories. Sometimes they'll die and their carcass will fall to an obscure area of the tank, where the other fish eat them before you can find them. My BN pleco is likely a big factor in that.

 

I haven't kept glowlights long enough for them to reach life expectancy yet.

I've had the same batch of glowlight tetra for 4 years now. Only lost 2 in that time.

 

I've had the same batch of glowlight tetra for 4 years now. Only lost 2 in that time.

Glowlights tend to be very hardy and resistant to disease of any kind. I have no doubt that they can easily live for 4-5 years. Mine aren't even a year old yet so they should have quite some time ahead of them. Rummy noses seem to have a life expectancy in the area of 2-3 years. All of my original cardinals have passed away so they must only live for a little over 2 years. I think in the wild they are annual fish and normally die at the end of their first wet season as adults which doubles as the mating season so double their wild lifespan isn't bad. Can't speak on neons as I haven't kept them for long enough but probably in the same boat as the cardinals. Saying that I imagine most tetras are annual and die in the dry season due to predation in the wild.

 

Glowlights tend to be very hardy and resistant to disease of any kind. I have no doubt that they can easily live for 4-5 years. Mine aren't even a year old yet so they should have quite some time ahead of them. Rummy noses seem to have a life expectancy in the area of 2-3 years. All of my original cardinals have passed away so they must only live for a little over 2 years. I think in the wild they are annual fish and normally die at the end of their first wet season as adults which doubles as the mating season so double their wild lifespan isn't bad. Can't speak on neons as I haven't kept them for long enough but probably in the same boat as the cardinals. Saying that I imagine most tetras are annual and die in the dry season due to predation in the wild.

I'm keeping cardinals now with my glowlights. I've had neons before and from what you've described about the cardinals, neons are definitely right around the same.

 

I've had the same batch of glowlight tetra for 4 years now. Only lost 2 in that time.

Have you checked your ammonia level lately? If you are finding skeletons in the tank it would cause a rise and therefore would cause more fish to die and deteriorate more rapidly. I would check the ammonia level as soon as possible if you haven’t already!

 

Have you checked your ammonia level lately? If you are finding skeletons in the tank it would cause a rise and therefore would cause more fish to die and deteriorate more rapidly. I would check the ammonia level as soon as possible if you haven’t already!

I'm not th op. Thank you though.