Ok, just checking.
How long has the fish been being watched by the person? And was the water kinda brownish and dirty then or just recently since the sitter has it?
That fish, any of the fish needs NO MORE FOOD for a little while. The other tank is dirty too.
All the water needs changed but if it was dirty when the sitter got them it should be done gradually maybe over a day or 2 with a number of changes as to not shock the fish. We don't know about the water where the fish came from and where it is being kept at, meaning the location/pH. A bottle of Prime water conditioner about $9. NOW!
This really is an emergency.
Follow directions on the bottle. Need a 5 gal bucket.
I would scoop out the food in the top with a cup.
next scoop the water out gently hopefully not scaring the fish too much and take out half of the water from the top part of the aquarium into the bucket.
Then add Prime to a temperature matched tap water to double the water - half old water half new treated X 5 with Prime the X 5 is the emergency dosage, for the amount needed for the size of the tank.
The 5X dosage is for the entire volume of the water in the tank and going into the tank- if the tank is 10 gallons then treat for 10 gallons total which includes new and old water.
Dump out and clean the water and bottom stuff from the fish tank in the pic.
Add the fish and treated X5 water back into the tank.
Remove 50% of the water from the tank, and repeat the temp matched water in the bucket treated for that volume of water with Prime X 2 again. This will be fresh water going in.
Add back the water into the tank slowly as to keep the water for the fish as calm as possible. Also if there is a light turn it off and let the fish relax.
This is an emergency back to back 50% water change.
Then the next day or somewhere 15-18-24 hours later do the same back to back pwc.
By then the majority of water should be exchanged.