Hi and welcome! Have you read through all the info on
www.redearslider.com? That's a good place to start learning about the proper habitat/feeding of your new friends. I guess you've already learned your lesson on this, but never trust what the people at the pet store tell you!
I think the first things you should look into are thermometers, water heaters and a proper basking setup. They need to be able to get out of the water and dry off completely, they need it to be warm (about 10 degrees warmer than the water) and they need UVB lighting. Is the basking bulb you have red? If it is, it should be fine, but in the future you can just use a regular incandescent bulb to heat up the basking spot. If it's purple like a black light, don't use it as it can damage their vision. You also need to get them a separate UVB light over the basking area, they are always a flourescent light (unless they are a MVB but i'll get to that in a minute...), it can be a compact flourescent that fits in a standard socket of a tube fixture. When you look for one, get one that's rated 5.0. 10.0 is usually too strong and 2.0 isn't enough. For your larger tank, you have the option of using a mercury vapor bulb (MVB). They are expensive ($40-50), but they provide both heat and UVB, and they last a year whereas flourescent UVB lamps only last 6 months.
You'll want to get good filters, but if you can only get a few things right away i would worry about the heating/lighting first, as you can compensate for inadequate filtration with frequent water changes. You might want to pick up some anacharis (it could also be sold under the name elodea) at the pet store, it's an inexpensive aquatic plant that's great for turtles to munch on. If you haven't yet, try offering them lettuce and other veggies. Look at the sticky in the feeding section of the forum for what's good to feed them and what's not.
About the legality of turtle sales in Texas, i'm not sure, but they are frequently banned/restricted because of salmonella concerns. Not to say that all turtles have salmonella, but they can carry it so do wash your hands after handling the turtles or coming into contact with their water.
I hope this was helpful, i'm sure you'll have lots more questions, so feel free to ask. We're all happy to help out new turtle owners. Good luck
