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i have a 3 year old res and im soon to be getting a rubbermaid 300 gallon stock tank that will last her for ever and 300 gallons is alot and i think one or more turtles could be with her but i dont know if i should go with more res or some thing else what would my turtle be ok with?

Unless you're okay with the possibility of having to divide the pond in two or house the second turtle in a separate pond, I'd say stick with one turtle. Time and time again regardless of the space given RES prove to be pretty aggressive and if yours is aggressive he'll need to be separated from the other turtle for their safety.
Also, the new turtle should be quarantined for 60-90 days in case either turtle is ill... do you have a place to quarantine a new addition?

If you're ok with the chance that they may not get along and can provide a second turtle home I'd suggest looking into adopting a turtle. There are so many that need good homes. If you do get a new turtle, try to find one similar in size. If there is a dramatic size difference the little one will easily get killed.

Also, if you are set on putting them together, it may be best to set up the stock tank, get the new turtle and undergo the quarantine period, and then add both of them to the new tank. If one turtle has established himself in the tank, he will be more aggressive. But if both of them are brand new, it will take them awhile to feel comfortable.

ya im all set for a quarantine but is there a way to tell if my res will be agressive

no. But, it's a safe assumption to assume that they will be. RES are just aggressive by nature, and most of them like their own space.

really, i have seen a lot of people who have very non aggressive red eared sliders who live peacefully with alot of other turtles and in the wild water turles are always around each other. and what turtles have the similar requirements a d would do well with res's

Many other breeds of turtles will do fine with your RES. It's your RES that won't do fine with him. And in a pond, 1) you don't know they do well, you can't see little dead turtle bodies at the bottom of the pond, 2) there is no competition for resources, there is lots of water and lots of possible food sources. Many times the people who have turtles together have lived together for years and have lucked out. It's not absolutely sure that two RES won't get along, it's just always a possibility. You can look for a cooter or yellow bellied slider, I guess they have similar requirements. You can always separate the tank in half if necessary, maybe keep a plexiglass or egg crate on hand just in case.

what about if i just stuck with red eared sliders and had them in the 300 gallon pond and made sure they were all pretty much the same size. would that be my best situation if i were to keep turtles together with my current res?

You can always try as long as you are prepared to separate them -- RES can be more aggressive than comparable turtles.