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I have a 10 gallon fish tank with two 2-year old male turtles (didn't know both were male when bought them) and some gold fish (bought 12 them as food for turtles, 7 remaining after 2 years and my turtles won't be able to catch them).

I have a small fish tank filter but I found it very insufficient and I have to change water for the whole tank every couple of weeks.
One big reason that I haven't upgraded my fish tank is the amount of labor involved in changing water and cleaning ...

Today I found that I could buy some better filters to significantly reduce the labor of maintenance. But I couldn't figure out which one will be the best fit for me. I would want one that's cost effective, low maintenance fee and have the potential to work for bigger tanks. I would probably upgrade my tank to 20 gallon in the near future.

Could you recommend some filters for me?
Thanks!

Two 2 year old RES and gold fish... you're going to need a MUCH larger tank than 20G. How big are the RES today?
The rule of thumb is about 10 gallons of water per inch of turtle shell. So let's say your turtles are 2" long each... that's 40gallons of water + goldfish. So a 55G+ tank would be appropriate. As the male turtles grow, they could get 7-9" EACH. The problem with turtles is that they might fight as they get older and need to be separated into separate tanks.

Larger fish tanks are actually easier to maintain IMO than smaller ones. There is more water volume in a larger tank so the waste (ammonia source) can dissipate. You do need a very good filter on a large tank, but the big tanks are a bit more forgiving (just not on your wallet).

They are about 4 inches each.
10 gallons of water per inch is too much to me.
I have seen recommendations of the Rena X3 on this forum a lot. But I am afraid it's an overkill for my case.
Are Rena X2 and X1 good? How about Zoo Med models?
Thanks

The X3 is going to be small for you. With two turtles I'd start getting used to the idea of a X4 or Fluval FX5 and a very large tank.