20 Gallon Tank Panda cory looking thin and not seen eating

I had a brand new 20 gallon tank set up for a week with 1 betta, not cycled and doing a fish in cycling (its terrible). After a week, I decided to get 6 corys and 2 female guppies to add to this tank and move the betta to a 3gal of his own. The corys and guppies have been in the tank for 3 days now and are doing well (eating and exploring all over). My water parameter isn't the best but here they are:

• Temperature: ~79F
• PH: 7.6 (when tested with API low PH test), 7.4 (when tested with API high PH test)
• Ammonia: ~0.25 ppm
• Nitrite: 0 ppm
• Nitrate: about 0 ppm

I originally had 1 panda cory for about 2 weeks in another 20gal tank (not properly cycled either) and moved him to the new 20gal tank to be with more of his kind. There are now 7 panda corys. Everyone is eating and happily swimming around exploring or shoveling the gravel except for the cory from my other tank. I've been keeping an eye on him and has not seen him eating. It has been 3 days since he is in the tank and all he does is lay on the gravel bed alone most of the time and rarely school with his new buddies. He is smaller and darker than his new friends and is looking skinny to me. I don't see any external signs of diseases. You can still see his barbels in the picture too.

I am new in fish keeping so would greatly appreciate any inputs or suggestions in what I should do to make sure this guy isn't starving. Would love to know if panda corys would typically take a while to adjust to new tank mates and tank conditions. The other 6 corys adjusted very well and was happily eating the sinking wafer within 2hrs of being in the new tank.

I have been testing the water twice a day and doing water changes everyday and condition the water with prime + stability.

 

Cories are really sensitive fish IMO especially in regards to water quality. The ammonia is most likely the issue. Putting more fish in an uncycled tank was not a good idea to be honest with you. But I'm not here to be overly critical, you already have them in there so we might as well try to save them. You're doing good by doing daily water changes with prime. Continue doing that and testing the water daily. I would start double dosing the entire tank volume with prime every day. Don't just dose the new incoming water. Prime binds ammonia and nitrite into less toxic forms for up to 24 hours, so by double dosing daily you are doing as much as possible to lighten the level of toxicity in the tank. Adding an airstone or two to up the amount of dissolved oxygen can also help fish deal with ammonia and nitrite poisoning.

Cory cats seem to also be really sensitive during shipping and new tank acclimation in general. I no longer keep cories but kept pandas a few times early on when I got into the hobby and if you aren't careful with them they are finicky. I had a lot of losses with them due to lack of experience but also from shipping and transport. I've even lost them due to stress in QT.

The last thing I want to mention is your tank temperature. Panda cories high end is at 77F, so the tank is too hot for them. Their range is 68-77F but I find they are most comfortable in the mid 70s. 74F is a great middle ground for them.

 

Is it bad If I accidently add too much prime?

 

No but it will lower oxygen levels if dosed excessively. So just double the dose for 20 gallons. Seachem had a chart on their FAQ part of their website showing how much prime to overdose for varying levels of ammonia, so seachem themselves stated it is safe to overdose in moderation.

 

Perhaps the other cories are simply neglecting to lone cory and taking all of the food for themselves. You introduced a single cory to an already established school of them, which also happen to be bigger and look slightly different. It can be hard to make friends when moving to a new school.