Wow it was 3+ years since the last post, glad to say my RES is still alive and begging. I have the same sized tank as you and my water level is filled to the 20cm height mark.
I have been using the same configuration in the Atman DF-1300 (refer to post #3) during all this time and glad to say it works well and I'm satisfied. I've only done filter replacement and/or cleaning after every 9-12 months. I did them not because the water became visibly dirty, but rather that I thought it had been quite a while. Therefore filter replacement and/or cleaning really could be an annual affair.
My water replacement regime is to drain all of the tank's water, leaving only whatever water is left in the Atman external canister filter after every 3 months or so. Again, I do water replacement only because I thought the water was becoming too yellowish for my liking. I used to add anti-chloramine to the tap water whenever I change or top up water, but for the past one year, I've tried just adding tap water neat without any additives and there was no material change to the turtle's health or noticeable water quality differences.
I don't use the rain bar function on the filter's outlet, instead the outlet is introduced underneath the water surface(at approx 10cm mark from the bottom of tank) so that it makes no water noise on the inflow.
http://www.ebay.com/itm/Pet-Fish-Aqu...item51c515a5e5
Finally, I highly recommend you use a surface skimmer hooked up to your filter's intake. The link above is exactly what I am using, it should cost you around SGD10 or the low $10's. Minor modifications were required, since I run my water level at the 20cm mark, I had to saw off about say an inch off the skimmer's black and clear section (the part where the 2 parts meet), such that it could function even when my water level evaporates down to say 16-18cm mark. The turtle sheds skin quite frequently and this together with other floating fine opaque bits that floats on the surface cannot be removed without the surface skimmer functioning. This is the best and only tool you will need to ensure water clarity - the external canister alone is not good enough, you will want both working in conjunction. Just remember to turn off the filter during feeding time and back on after they are done eating.
The Atman DF-1300 was purchased at NA, this is their FB page
https://www.facebook.com/nanatureaqu...?tab=page_info
Not certain whether this is still an ongoing production model, or whether the $95 pricetag remains, but I would still stick to the same brand as it has been great value for money and would go with an identical sized unit currently offered as the DF-1300 assuming it has been phased out.
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