That's quite an assortment of critters!
With all of those different fish, I think you're much better off leaving the pH where it is. Some of them like more acidic water, and some like more alkaline, so leaving it near neutral is a good compromise.
From those pictures, fish I saw that you hadn't explicitly listed were a powder blue dwarf gourami (this could be your blue fish) and some sort of dark colored catfish (is this the one that looks like a jet?). It was sort of difficult to see that fish in the pictures so properly identifying it is tough. It could be some sort of whiptail cat, possibly.
Just as a word of warning: that clam is likely going to die, since they are filter feeders and really need a steady diet of green water in order to live. Some members here have suggested trying to spot-feed clams with liquid fry foods.
Gump, one of our more experienced fishkeepers, had this to say:
...Clams are filter feeders so if you don't have a lot of micros in the tank they wont last long. It's hard to tell when a clam dies and if you leave a dead clam in the tank for over a few days it will muck up your water quickly.
One more thing: I definitely wouldn't recommend adding a turtle to that tank. It's already quite heavily stocked as it is, and aquatic turtles add a ton of waste to your system. They also require a basking area above water as well as special UV lighting to keep them healthy. The most commonly available aquatic turtle, the red eared slider, gets to be about a foot across and needs a 75 gallon tank. The smaller musk turtle could be kept in a 55, but not with those other fish. Last, but not least, any aquatic turtle will eat live fish so the fish you've got in the tank now wouldn't last very long.