self sustaining ecosystems

Question
I have a pair of male Red Eared Sliders, about 5-7 years old, and I want to set up a pond for them outside. Currently they are in a 50 gallon tank, with a floating basking spot and fed only pellets. I would like to create the most self sustaining ecosystem I can for them. We're in Phoenix AZ, so the weather can get up to 115 F in the summer, and maybe 3 days of 29 F in the winter. I have a spot picked out, and will have at least 2 feet of wall above ground and 8 inches underground to avoid them tunneling out. I have a preform pond(around 150 gallon), pump and bio-filter. I have read up on water plants to add to the pond. If I add minnows or other feeder fish to the pond , will they be able to breed, or will the turtles just eat them all the first day? Are there land plants that I should use/avoid, or will the turtles just ignore them? Should I add a heat lamp or water heater for them in the winter, or will the sun be enough? Do you think the turtles will like the fountain part of the water pump, or would they rather have still water? (I'd like to avoid breeding mosquitoes). That's all of the questions I can think of. Any other advice you can give me would be great. Thanks!

Answer
Hi Terri,

I have limited pond experience, so you may need to talk to a pond specialist in your area on some of this, but I can address the turtle aspect.  First of all, if you want your pond to be as self-sustaining as possible, I think you need to make it bigger--at least double the size, and 500+ gallons would probably be better.  If your turtles are about 6" each, they'd need to be in at least a 150 gallon tank, but that's with a heavy-duty filter and regular water changes/cleaning.  For a partially or fully self-sustaining pond, you need more room or you'll get too much waste build-up.  

Another issue is water depth.  Your summer heat means that the water will get very warm, so there will need to be areas where the water is fairly deep so the turtles can cool off sufficiently.  How deep, I'm not sure, and that's one thing I'd ask a local pond specialist.  You'll also want to make sure that there are areas that are heavily shaded to help cool things off.  

Winter cold days shouldn't be a problem.  The turtles will hibernate if it gets cold enough, but if it warms up sufficiently during the day (especially if it's sunny), it may be only for a short period of time.  I've seen RES in a local pond out basking when overnight temperatures where in the 30s and daytime temps in the 50s.  However, you should only let them winter outside if they are in good health, so in the fall inspect them for any signs of illness, infection, etc.  

If there are plenty of plants and hiding areas, the fish should be able to breed.  The turtles will eat them, of course, but there will be survivors.  They will also eat the pond plants, and may do some rooting around as well.  As far as land plants go, don't plant anything toxic.  If the plants are edible, the turtles may eat them, but probably not to the point where they destroy them.  Here's links to some safe plants, although you'll have to take the heat into consideration:

http://www.tortoise.org/general/edibplan.html
http://africantortoise.com/edible_landscaping.htm

And a toxic list:
http://www.chelonia.org/Articles/plantsthatpoison.htm

Having a fountain is fine, and fact with the heat is probably a good idea because it may help to cool the surface water off a bit.  The fish will also help with mosquito control.  

Here's an article that may be helpful to you.  The Tortoise Trust is a reliable source of information, but keep in mind that they're in the UK and have much cooler temperatures than you do:  http://www.tortoisetrust.org/articles/naturtles.html.

Good luck!