I recently added 3 male guppies to a moderately planted 20 gallon long tank (48 hours ago). On the first day, all three roamed around the tank and acted very normal. The following day, one was spending a lot of time right up against the surface of the water. His fins move, but he stays in place. I tested my water, and everything looked fine (ammonia 0ppm, nitrite 0ppm, nitrates 10-20ppm), but I went ahead and did a 25% water change to be safe. I did not notice a change in this behavior. Today, two guppies are now exhibiting this behavior. The third occasionally comes to the surface, swimming near the other two but eventually going back to roaming around the tank.
I'm not sure what's causing the guppies to act like this. My tank is cycled: ammonia levels never reach or go above 0.25ppm, nitrites are always 0, and currently nitrates are 10-20ppm. I can confirm its cycled since my nitrates creep up over the week. The temperature fluctuates is between 77-79F, pH is 8.0, gH is 13, and kH is 9-10. I bought them from a LFS that uses the same water source and has very similar parameters.
At first, I thought it might be an issue with oxygenation. I have a HOB filter (Aquaclear 30) which is meant to handle up to 30 gallons. I have it on max output, so all the water on the top of the tank is circulated. I can see bubbles in the column where the water falls into the tank from the filter. However, the guppies are not gulping for air. Additionally, there are 6 pygmy corydoras in the tank as well, and they do not go to the surface for oxygen more than what is considered normal--they're actually quite happy and active! So, I'm not sure oxygenation is the issue.
To try and test if oxygenation was an issue, I've tried lowering the water level while the filter is on max to try and get more gas exchange at the surface. This did not change the guppy's activity after an hour or two. Then, I switched it to a lower flow, just in case that might be what was bothering them. This also did not affect anything. The last thing I did was gently squeeze/swish around my filter media into some of the tank water I removed to lower the level to try and clean it a bit to see if that allowed better water flow and improve water conditions. This has not done anything either.
I'm not sure what else to try. Does anyone have any experience with this and have suggestions on what to do? It's puzzling given that my pygmy corys and amano shrimp are happy and active, seeming to be just fine.
One last note: on the first day, one of the guppies was fin-nipping. This has substantially decreased, but the stress of this may be affecting their behavior. The two guppies that are hanging out at the top are the aggressor and the victim (the third does his own thing for the most part, but often hangs around the victim). The aggressor was from a male-female mixed tank, while the other two were from male only tanks.
Update: I just turned off the tank lights as I normally do at this time of day, and now the two guppies are acting normally. I wasn't under the impression that the lights could have an effect--what's going on here? I don't want to keep the lights off or go lower in intensity since my plants and algae management are working really well with this lighting level/schedule. What should I do to make them feel more comfortable with the lights on? A lot of my plants are still short/young, so could it be that they don't feel like they have enough hiding places with the lights on?
Last edited by Darkblade48; 09-25-2019 at 12:42 PM.
Reason: Please use the edit function for back to back posts to keep threads cleaner