Thank you for the answers , helps a lot , so I will try to be clear with reply.
As I stated not all turtles respond the same to things even from the same clutch. That one turtle from your info sounds like it has a cold. Is very contagious so separate them ! That one may of been a little weaker than the other when you got them. It's feels the temps being off more . Correct the habitat and hope that’s enough to turn things around before it goes into an “RI” respiratory infection . That’s hard to cure in babies. I understand you did do some reading on habitat’s But your setup is more for an adult turtles needs. Babies RES’s turtles needs / requirements are different. It changes at every stage of a turtles growth.
Like one person in a house catches a cold and the other does not. But can and will spread in a small setup so separate the turtles for prevention.
1)Water temp too cool for babies. That one feels it more than the other one. Yes only a few degrees off but that’s a lot for a small baby turtle. Not a lot to a human but off to a baby turtle. Water needs to be a consistent 80* day and night ! Also just because a water heater dial is set on 80* does not mean it's reaching that temp. And that one you can not calibrate it. Main reason for a very accurate thermometer !
2)Those thermometer’s are known to be inaccurate. Look at ten in a store and all are different temps. Look into an infrared type , most accurate and can be used on everything ! Basking dock true temps , water and even room temps , and lots more uses. Your reading’s may be off more than you think. Also for babies basking dock needs to be at 90* . Not 90 watts. Study up on what bulbs do both UVA and UVB and see all the different distance they , each wattage , is recommended to work properly for a setup. The light you see is not all UVB rays , some even at proper distance only have usable UVB ray’s in a few inches so placement “very” important !
3)Need to know waters parameters , will be your best friend ! Study up on cycling.
http://www.oscarfishlover.com/how-to-cy ... r-aquarium4)Since you say N/A on basking temp I have to assume the (90) you say is the wattage ? That solar glow does both UVA and UVB but did you read the specifications on placement ? It’s only has use-able UVB rays at a certain distance. Different wattage s are for different size setups.
https://www.bing.com/videos/search?q=UV ... ORM=VRDGAR 5)Move bulb or basking dock a little so the solar glow is at the center where they can dry off better , that after you get a thermometer to read temps on basking dock so you know the distance for placement. Read the spec's . Looks a little too far away now for a solar glow type to me.
6)Diet is not bad but try some mazuri or omega one , much better.
Info on the internet is confusing especially when they don’t say what size turtle or species they are talking about. You are not far off but enough off to were you turtle has a cold. Correct habitat to what a baby RES turtle needs and that may help correct it’s health. If not will go into an “RI” respiratory infection ! That’s hard to cure in baby turtles and can not be done with “OTC” drugs , a Herp Vet is needed for “RX’s” ! Not doing bad for a beginner just tweak some so your turtle does not pay the price and observe well . Better habitat , especially the water temp and basking temps will help that one feel better with those other tweaks. How about using that long flat driftwood as a basking dock , would offer a much better range of temps than a small dock that's always hot. Let your turtle pick temp it wants or needs , sometimes they just want to dry off without all the heat.
PS; those tank dimensions are a 20 gallon high in the US. Won't be long they out grow that , next tank get a long style , RES's like to swim left to right not as much as up and down. Better hope both are not females , at adulthood will need 150-200 gallons water alone.