Disappearing Fish

I have a lightly planted 20 gallon tank, gravel substrate with a small bristlenose pleco and six harlequin rasboras. Have had the tank for a year, all was fine and the fish were doing great. All of a sudden my harlequin rasboras have started disappearing, one at a time. I don’t think the pleco would eat them, he mostly eats algae pellets and some of the driftwood in the tank. I’ve checked the filter, not there, and the tank had a lid so they can’t jump out. I thought maybe they died and decomposed before I realized, but i have 4 yesterday night and when I looked at them today there were only three, with no trace of the missing one. Could it have been decomposed that fast? Tank parameters: ammonia 0, nitrites 0, nitrates 40 ppm, pH 8.5

I don't see a fish decomposing that fast unless something ate it after it died or was eaten when alive. Do check under hardscape too I've had a couple of fish manage to hide in small gaps. I have little to no knowledge of pleco behavior I'm afraid. Other members will have a much better idea than me.

@CassCats Can Pleco's turn to the dark side? (omnivore)

MattW said:

I don't see a fish decomposing that fast unless something ate it after it died or was eaten when alive. Do check under hardscape too I've had a couple of fish manage to hide in small gaps. I have little to no knowledge of pleco behavior I'm afraid. Other members will have a much better idea than me.

@CassCats Can Pleco's turn to the dark side? (omnivore)

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100% the pleco will most definitely eat the bodies and harlies bodies being as small as they are will be disposed of very quickly, gone even by the next morning.

Thanks so much! Checked under the log and nothing, so I am forced to assume my pleco has turned evil and eaten my fish . Is it normal for harlequin's to die after just a year? I got them from a non-commercial fish store (not PetCo etc) called Wet Pets and Friends (located in Pittsburgh). They have good quality fish, and told me they live around 4 years. Is this accurate? What can I do to get them to stop dying??

What are your water parameters, such as pH, GH, and KH?
Your nitrates are high, which longterm is stressful on fish and can easily take out more delicate fish.

Solution for the high nitrates is more frequent water changes, they should be done every week. Nitrates should be kept below 20ppm, ideally even lower. If they hit 20, it's time to do a water change.

You also can add more plants to use up extra nitrates.

It's not that the pleco has gone "evil", just doing what they do and eating what is available to it. They do eat some protein and will not care if it is fish food or a fish body. Some plecos actually need more meat than vegetation (bristlenose not so much, but they do need some meat to their diet anyways)

I'm a bit concerned about your rasboras. They seem a bit skinny & small especially if you've had them for close to a year. They should be adult size or almost.

How often & how much water do you change? That's my first line of defense for problems. It can make a huge difference!

A side note, a dark tank background would help your tank aesthetics. Then your filter would not be so visible & fish would look brighter.

I usually change about 25% of the water every two weeks or so, I know it should be weekly but I can never find time. Since they started disappearing I've been changing 1/4 every week though. I'll try a dark background, thanks!

Is there a lid on the tank?
A lot of fish feel more secure with overhead plants. Have you tried any floating plants (frogbit, salvinia, water lettuce, red root floaters)?

I had the smallest Harlequinn Rasboras I've ever seen arrive July 17th and the 3 that are left already look bigger than yours. Something doesn't add up there.

>>> pH 8.5
Your pH seems awfully high for heteromorpha to thrive, you might want to think about lowering that. They might survive for a while, but certainly not thrive.

Just had an unfortunately timed week long vacation where I had to leave my fish; I left them with a five day food pellet and changed 25% of the water the day I left. Fortunately all the fish survived, and the entire food block is gone (I’m thinking the pleco probably ate most of it since he’s bigger, but the harley’s look much healthier too). Water params: ammonia 0.25, nitrite 0, pH 7.4, nitrates 30ppm (kind of high, but it’s been a week since the water change). However, my big anubias plant has gotten some kind of rot attacking it, its leaves are turning brown and mushy. Any way to fix this? It happened to my octopus plants a month back and now they’re almost gone.