Hi,
I too have a couple of watergardens/garden ponds. We have a couple of painted turtle clutches a year. In fact just found one sunday. 7 turtles in this nest. The adults live year round in the ponds here in kansas. Drawbacks for the small ones are a few. The koi can definitely eat them along with bullfrogs, although I don't think they are a problem in washington state. The other main problem is your filter system. If you have a skimmer filter, that is worse than any predator. I usually have had mine hatch in late summer and overwintered inside. But a couple of months of feeding and growing and they should be fine in pond. A bog area can help speed things up because they will normally stay in there. Hope that helps.