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Squirt does need a light source--both a light to heat the basking area (can be a regular lightbulb) and a UVB light (to metabolize Vit D3 to help with calcium absorption); a basking area temp in the range of 88-90F is good.
He has a submersible water heater? Since he's so small, it would be good to up the water temp to about 78F (a good temp for hatchlings). Since he's in a 10-gallon and swimming space is precious, instead of rocks (which take up space) you might consider a basking area that will allow for swimming space underneath (a number of keepers and their turtles seem to like turtle docks).
A good aquatic plant to start Squirt off with is anachris, but it would be good to also start introducing him to pieces of leafy greens such as red leaf lettuce, dandelion leafs, etc. (Check the info here for what's good to give). What brand pellets is he being fed? How often is he fed and how much is he given at a feeding? (At his size, for pellets, once a day an amount that would fit into his head is a good guide.)
Does he have a water filter?
You won't be able to really sex him until his shell length w/o the curve is more like 4 inches.
"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-