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i am thinking of buying an XP filter for my 40 gallon tank. i've seen on previous threads that it is the best filter to have for a turtle. but im not sure if is hould buy the XP3 or XP2. i would like to go with the cheaper option, but if the XP3 is better, then i might as well get that. any suggestions???

Well, remember that you'll have to upgrade your tank sooner or later, so you may as well go for the xp3 now, otherwise you'll likely have to buy a new one when you get a new tank...

Your other option is to buy the lesser model and suppliment it with a second filter after you upgrade. There are pros and cons to either method and it depends on your needs and what you want.

A XP2 or XP3 is overkill for a 40 gallon tank, I think. Like missibsu said, get a less powerful/cheaper filter for now.

How much water will be in there?

I use the XP3 with a 40-gallon stock tank without any problems. Use the spray bar and angle it toward the wall to lessen the current if that's a problem. I'd get the XP3.