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Hi everyone,

So this is going to be a lengthy explanation but the gist is this: I have a 5 gallon freshwater fish tank that I've had for about 3 months. In it I have all the original fish that I got with the tank setup: 3 neon tetras, 3 rummy nose tetras and a yellow gourami. They have been happy and healthy up until the last week or two when they ALL have started having weird symptoms such as: losing all of their colour, becoming erratic (darting, shivering, twitching), separating in the tank, drifting backwards etc. Basically they are stressed. But the weird thing about it is that it only happens at a certain time of day; that is, from about 9 or 10pm till the early morning (at least as late as 4am). They are perfectly fine and happy again when I wake up in the morning.

It is also worth mentioning that I have 5 small cherry shrimp and a handful of snails in there that are NOT affected. I've been told that the shrimp are more sensitive to water quality changes than my fish so it's bizarre that they have been unaffected.

I know it is something in the water because when I do a water change they return to normal within 30 minutes. This is NOT a behavioural response due to one of the fish bullying the others. I have taken a water sample into a reputable, knowledgeable aquarium specialist where he tested for: Ammonia, Nitrite, Nitrate, pH, Hardness (Carbonate and General) and Copper. All the tests came back perfect. Additionally I make sure to always treat water I add to the tank to remove chlorine/choloramine and add a standard biological aquarium supplement, as well as a supplement for the plants. I have LOTS of plants in there as well as rocks (no coral, but a few small shells as well) so oxygen is not an issue (they aren't gasping at the surface anyways). I have changed the carbon filter recently, keep the temperature stable at 78F and feed them moderately once per day with crushed tropical fish flakes. I have also switched their feeding and light (when I turn on/off their light; usually it's on ~12 hours/day) schedules to see if there is a change in behaviour or time when they become stressed (also diminishing the light slowly rather than abruptly). This hasn't changed anything, they still always become stressed at the same time and in the same way.

There is also no way that anything is getting into the water from air fresheners, etc. etc. The onset of the symptoms wasn't correlated with any changes to their habitat (eg. addition of rocks), the fish/invertebrate community, or really any other changes; it's essentially come out of the blue.

My best guess is that there is some biotic or abiotic process happening at night that results in a release of some sort of toxin into the water, and that it is gone by morning. Does anyone have any idea of what this might be or what sort of tests I can do to isolate the problem? I'm pretty desperate here haha, I don't want my fish babies to be stressed and die!!

You all are awesome, thanks so much in advance,

Justin

P.S. I will attach a file tonight showing the stress response.
 

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