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I could really use some advice. I have had one of my red eared sliders since 1998. I got my second in 2002. Even though the second is much younger and smaller they have always gotten along. We moved this summer and purchased a much larger tank. After about 3 months, the older turtles did nothing all day but swim after the smaller turtle. The smaller turtle would fight back but eventually becomes exhausted and the older turtle bites him and won't let go (neck, arm, or tail).

We ended up going out and buying a second tank (we gave away our old set up) and they have been separated for over a month now. Every time I try to integrate them we get the same results.

Does anyone know why this is happening? Is there anything we can do to get them to live peacefully again? I really appreciate any advice you can provide.

You did the right thing by separating them. Unfortunately they'll have to remain separated for life, once turtles start aggressing it usually only gets worse from there. If you were to put them back together there's a good chance the smaller one would be seriously hurt or killed.

I was afraid you were going to say that. Is there anything wrong with him? I have had him for almost 10 years and he just seems to have gone crazy. Is it behavior that happens with age?

It could be the introduction of the new tank, sometimes it sparks territory issues. Really turtles are solitary creatures so it most likely was just that the turtles have finally had enough of each other, there's nothing wrong with either of them.

I noticed that when I moved my younger RES and MAP into a new larger tank, that is when the aggression started. I eventually had to seperate them permanently.

wierd that they weren't territorial in the smaller tank.......

Interesting, my two RES where fine with each other in there 29 gallon long, but they started getting territorial when I upgraded them to a 240 gallon tank.

Unfortunately the fighting behavior you described is fairly common among RES's. They are naturally more aggressive than some of the other "painted"-style turtles. You did the right thing separating them into their own tanks.

Actually, putting two turtles in an enclosure that's new to the both of them is recommended when integrating turtles into a tank---it's unfamiliar and is the territory of neither. Of course, either can attempt to establish it as his, and then trouble starts.

Just curious, how big is the new tank? Were there objects, plants, etc. in the tank to interrupt their sight lines so they weren't always visible to each other?

If you've been keeping the smaller one in the new tank and periodically are attempting to put him back in the larger tank with the larger turtle. The latter is just viewing him as an intruder...Now that you've separated them, keep them that way.

I think i might be extremely lucky
To what I've seen, its really only the males who fight over territory. When i have two males alone in a tank, its pretty much inevitable eventually. The thing is when my larger female is with them, they completely forget about eachother and focus on her (wanting to mate, not fight). Shes very docile and really just swims away unless shes accepting it. The ony fighting i see is when she overwhelmed by them and bites once (or really fakes it, opening her mouth to suggest shes gonna bite) to get them to swim away and it works.
A rule I go buy is that there should be enough space so that the turtles wont be able to see eachother all the time. In my setup theres a dark crowded corner of wood/plants and also a large area of land where they can completly get away. The area of land is much more a preemptive measure, they never jump out of the water if she nips them, they'll use the corner, but when turtles are out of the water they wont see eachother and this gives them their space i guess.
I havent seen any fighting in a long time and since ive put 5 turtles in a tank together there hasnt been a single issue!