Fish with red gills and acting strangely for unapparent reasons

[Tacocat]

When I checked on my fish about 20 minutes ago I found them with red gills and concerningly unresponsive to my hands. I quickly did a 50% water change and almost immediately after they started looking more normal and acting better. An interesting thing I saw was that the numerous caridina shrimp in the tank did not seem to be bothered at all by this ordeal and were all behaving normally. I feared that I hadn't done enough water changes as I had done my last one about two weeks ago. I had been regularly replacing about 1 litre of water a day though, so I thought that would be enough. This doesn't seem to be the case however because the shrimp are fine, and that all the fish were normal for the past month up until I checked earlier. I can't think of anything that I added to the tank recently other than some regular glass maintenance with algae scraper and me turning off the CO2 for the night. Any ideas at all are appreciated.
Yes the tank is cycled.
The fish are 5 pseudomugil gertrudae and 2 common otocinclus, and the shrimp are 10 caridina blue bolt

Here's a picture:

 

[Colin_T]

Carbon Dioxide (CO2) poisoning from supplemental CO2, lots of plants, and no aeration at night.

 

[Tacocat]

Carbon Dioxide (CO2) poisoning from supplemental CO2, lots of plants, and no aeration at night.

It’s been fine for weeks though I don’t think so. Plus I turned off the co2 for a while before night came. The filter was also running normally so there was definitely aeration

 

[Tacocat]

Colin you may be correct because the fish are fine now and nothing seems to have changed. I think the co2 meter was also green last night when I checked so that’s interesting.

Thanks

 

[Colin_T]

Sometimes in heavily planted tanks the fish can run out of oxygen at night due to the plants taking up oxygen when it's dark. So even if the CO2 was turned off, there might have been some left in the water and the plants added to it during the night. Filter bacteria also use a lot of oxygen and if you have a lot of gunk in the substrate, or even a deep substrate (3-4 inches), you can have a lot of bacteria there too and they need oxygen as well.

If the water temperature was warm, that can reduce oxygen levels too. I doubt that is a problem in Canada at this time of year unless you have an aquarium heater set too high or a warm house, but we have issues here with high temperatures and low oxygen levels. To counter this a lot of people with plant tanks run an airstone at night and have it turn off an hour before lights come on in the morning. I just had a couple of airstones in all my tanks bubbling away furiously and never had any issues.

HOB (hang on back) style filters need the water level an inch or more below the filter outlet so there is splashing water. Even then they don't aerate the water anywhere near as well as an airstone blowing bubbles.

Sometimes a contaminant can get into the water from outside the aquarium, and this can reduce oxygen levels. A big water change and aeration usually fixes a lot of these issues and they are the first aid for this type of situation. But it's good to hear they have recovered after a water change.