I would think that you might be able to use a kiddie pool put in a hole
for the liner, this would be cheap($10). Getting good circulation with
the pump would be important. Creating turbulence in the water is
important for aeration and the microorganisms that eat the feces need
oxygen to do their job. Growing some plants in the pond would
definitely help filter the water, water hyacinths float and are very
good at filtering water. There are some wastewater treatment plants
that use them to polish their effluent. You could also collect some
plants for this since that would give you some plants that are happy in
your climate. I would suggest adding some 10 cent goldfish to the pond
so that you don't have to worry about mosquitos. The goldfish might
also further "process" the turtle waste. I created a simple cheap
biofilter using a plastic screen plant basket from home depot(found in
the pond section). I put some plastic scouring pads on the bottom of
it, put my pump on the pads, placed some plastic sponges around the
pump, and then filled up the basket with lava rocks. My pump outflow
pipe sticks out of the water around six inches, I covered it with a
piece of bamboo and the water fountain effect is pretty nice. Total
cost of this: basket=$2.50, scouring pads, sponges=$1.00, lava
rocks=$1.00, outflow pipe $1.00 for a total of around $6.00. Email
back if you have any other questions about the pond.
Have fun,
James