Goodness, there's so much to tell you. I guess I'll just start with your specific questions, and I'm sure others will chime in. It sounds like you're off to a pretty good start.
"The previous owners feed it mostly meal worms and the occasional lettuce."
-These are poor staple food items. Earthworms are going to be your best bet. They're good for the turtle and every box turtle I've ever had has LOVED them. Waxworms, superworms, crickets, roaches, etc. are all good to offer also. From the garden, slugs and snails are usually relished. At least every other day, offer dark leafy greens like kale or collards. Keep offering at least a few bites even if they're not eaten. You may be suprised one day, even a year down the road, when your turtle gobbles up the whole plate. Mix in a variety of other veggies and some fruit. Carrots, partially cooked sweet potatoes, squash, cucumbers, tomatoes, all kinds of berries, chopped apples, cantaloupe, are all eaten by my turtles. Some of these items are better than others, but all have nutritional value. Try not to offer only what your turtle prefers, as they can sometimes be like kids...they'll choose the least nutritious item on the menu.
"Also they did not have a light for the house. I have a UV light that clips on the side."
-What specific type of bulb is it? It's good that you're providing UV, but there are two types...UVA and UVB. Only a few bulbs provide both. You will want to supplement the turtles' food with some vitamins and minerals, but which type will depend on the lighting you are providing.
"Also it's nails are extremely long and the info I was reading said that was because of the substrate."
- It could be. It could also be dietary. Nails are fairly easy to trim. A Vet visit may be in order for the first trimming, and they can teach you how to do it from then on.
"In the house they had been using dirt(very sandy and hard)."
- Bad choice. Switch to a softer, moisture retentive substrate. One poster recommends "reptile mulch" mixed with sphagnum moss for an adult Three-toe. I think that's a great idea. As I keep my adults outdoors, I haven't had to mess with indoor substrate choices for anything but hatchlings.
"It has a large pool that it can wade in and a hollow log it sleeps under. What else does it need and does anyone have any suggestions to keep it happy and healthy?"
- Just to note a few...try to provide a warm end and a cooler end. Keep the water in the cooler end. Provide hiding spots in both ends. Shoot for around 70 degrees on one side and approaching 90 on the other side. Monitor the humidity. Constantly breathing excessively wet or dry air can be very harmful to a box turtle. A range of 60% to 75% is usually good for an adult Three-toed, as long as it can soak and hide in moist substrate. Also, they're prone to shell infections so keep an eye out. An acrylic tank is going to hold humidity. You may want to consider switching to some other enclosure. What are the dimensions, including height?
Depending on where you live, an outdoor enclosure would likely increase your turtle's health and happiness immensely!
"From what I read it has a light colored shell because it needs more UV light and the shell is somewhat curled up around the edges because the substrate is too hard. Also the house is large, but it was made of acrylic and I read that it should not be see through. Does anyone have any ideas hoe no make it opaque?"
- A lack of natural sunlight and dietary plant pigments can cause a "white-washing" of the colors. This is usually seen in the skin. A curled shell can indicate a past dietary deficiency. Do you have any pictures?
You can tape construction paper around the outside of the acrylic to make it opaque.
I hope this helps. I'm sure others will have great suggestions and answers to any further questions you have. Good luck. And again, I'd LOVE to see pictures if you have the means.