If you’re living in El Paso, keeping a turtle outside would be fine. The weather is warm practically year-round. You need to check with your local laws first about species allowed, zoning, etc., but if it’s okay, then go for it.
If you want pond turtles, you’ll need a filtration system to deal with the poop and the food debris. Also, absolutely no chlorine- they will become deathly ill from it and eventually waste away. Local pet shops are a good bet, but please, don’t shop at Petco, and don’t patronize a store where the animals are crowded are unhealthy- your turtle will likely be sick from such a place, and you’ll be supoorting a disgusting business. Look for a place with clean tanks, clean water, no crowding, and knowledgeable employees.
You’ll need to provide fencing for the turtles- about four feet will do, but check on the species- some can climb. Some also dig. Buy a bunch of books about turtles and try to learn as much as you can.
You can also keep tortoises, which cannot swim- if they get into water over chest level, they will drown, so I don’t know if this is what you want with your pond. Tortoises will also dig big holes in your yard. But they’re so damn cute!
Tortoises are usually strict vegetarians-(although hatchlings sometimes eat animal protein) feed them leafy greens and some veggies and fruits. They’ll also eat timothy hay and they’ll graze your lawn, so no pesticides. This goes even for pond turtles- chelonians cannot deal with chemical concentrations, it causes respiratory and gastrointestinal problems. Turtles are usually omnivorous, leaning more towards carnivorous- goldfish, earthworms, insects, and even mice for large specimens will be readily accepted.
But keeping turtles and tortoises, especially large ones, outside is a wonderful idea. Chelonians need exposure to UV light in order to synthesize vitamin D3; artifical lights work okay for smaller species. You will also need to provide shade for them- reptiles can and do overheat, despite their ability to maintain higher internal body temperatures than most mammals. A plastic dog house for the big guys, overturned flower pots for the little guys work pretty well.
Daily maintenance would include feedings, some poop-cleaning, and pond maintenance. Many chelonians learn to recognize their keepers and will come to the fence when it’s time to be fed. Some will even solicit petting by stretching their neck toward you.
Don’t take any animals from the wild- it’s more than likely illegal, and wild-born animals seldom adapt well to captivity except in very special circumstances.
Don’t keep different species together; don’t keep turtles of very diferent sizes together; and try not to keep males together, although females of most species seem to get along. (Males have longer tails.) My experience is mainly with Sulcata tortoises, with are very large chelonians (weighing up to 200 lbs. for a large male.) These guys dig huge holes, eat a huge amount, and poop correspondingly. But they are very docile and friendly, and quite intelligent. There is a harem of one male and three females at work, and one of the females likes to follow me around when I go in for observation.
Ummm…there’s so much more to tell, but my brain has left the building just now. Keep us posted, and good luck. There is so much info, good and bad, out there, I’m sure with just a little research you’ll know if turtles are right for you and what species and requirements are right.