I did water change and gravel cleaning today. Before that I noticed that one of my neon tetra is missing. And nowhere to be found. I noticed the same thing last night but didn't think much of it. Thought it must be hiding somewhere. But today I looked under every plant and moved everything to find that tetra. All this before I did the water change or cleaning. It is simply gone. Vanished. I don't know what to think.

After water change, I checked water parameters. pH/dH ok, hints of ammonia, nitrate and nitrite. I put the chemical (nutrafin pack) to counter those.

All my fishes are about the same size except one gourami, which is quite gentle. None of the fishes are violent. I simply can't think of anything that could make this one tetra disappear completely. I looked for about 30-60 minutes for it (or any remain).

Here is the relevant info:
10g aquarium
artificial plants, some rocks
Some guppies
3 neon tetras, now 2 :(
1 gourami
1 shrimp
pH/dH is in the green
Hint of Ammonia from water change
Hint of Nitrate/Nitrite either from dead fish, or from gravel cleaning, or from overcrowding, or all of the above.

Your Gourami may be peaceful, but in a 10G he might be the culprit. Another alternative is the fish jumped out, are there any spots for the fish to jump out of, is the water level high, and have you checked around the aquarium and on the floor for a dead fish? The Gourami could have chased him to suicide.

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30G planted aquarium with Aquaclear 50 and two Aquaclear 20s. 7 Cherry Barbs, 12 Cardinal Tetras, 1 Albino Bristlenose Pleco, 1 Golden Honey Gourami, and a handful of Ghost Shrimp.

I have never seen the gourami chasing other fish. He mostly keeps to himself in relatively shady areas.

The water level is not high enough for the tetra to jump out through the filter hole in the lid. Unless it can jump 4"-5" above water.

I have checked everywhere. No luck. It is a fairly small fish, a baby tatra, about 1". But not small enough to be considered as food by any of the tank mates. I'll keep looking.

I can think of only one other possibility: it could have been buried under gravel when the gravel was moved. Usually fishes move away when I am moving gravel for any reason.

Can you think of any other possibilities?

did you check inside the filter? seems wierd but I have seen very small fish get sucked in.

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20 gallon
12 apple (mystery) snails
1 Glofish
1 crown tail beta
1 japanes trapdoor snail I found in the farmington river
2 Emerald Cory Cats
1 large common pleco
1 rainbow shark
2 bamboo shrimp

20 gallon
1 young tiger oscar
1 chinese algae eater

10 gallon
Looking to get an apricot lobster

I had a quick look. Didn't really consider that before. Will check it thoroughly tomorrow.

Darn, forgot about the filter. My friend a while back had a massive (55G tall I think) aquarium with tons of guppies and their fry. Many of the fry got trapped in the filter and a month or so later when his stepdad went to change the filter he found around 10 growing, healthy guppies living it up in the filter. They must have grown up to be some good swimmers cause they were constantly fighting the flow.

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30G planted aquarium with Aquaclear 50 and two Aquaclear 20s. 7 Cherry Barbs, 12 Cardinal Tetras, 1 Albino Bristlenose Pleco, 1 Golden Honey Gourami, and a handful of Ghost Shrimp.

You would be suprised how high a fish can jump.

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http://bountifulbettas.blogspot.com/

"Come to the dark side....we have cookies...and filters/heaters/and water changes!"

The fish was in fact stuck in the filter. Unfortunately it didn't survive. Although I rescued a guppy fry from there. Judging by its size it was malnourished, but heck of a swimmer. I put a piece of stocking on the filter to prevent any further tragedy.
On a more interesting note, while I was setting up the filter the shrimp shed its shell.