Hello.
A submersible filter is always a good thing. It allows for less water changes and is better for the snapper.
Your take setup sounds pretty good to me. They really do like hiding places, so make sure you have one. I created an underwater cave for my snapper using two rocks placed on the bottom of the tank and a slate rock on top. My snapper spends most of his day either on top of the slate rock basking or inside the cave.
Substrate is not necessary but is nice to have. I use a sand/gravel mixture that my snapper really enjoys. He digs a lot and likes to bury himself. Also add some plastic plants if possible. The more ways for the snapper to feel protected the better.
If you will be keeping your snapper indoors I would not worry about having a heater. If you notice that your snapper is not eating well and is rather sluggish I would recommend adding one, if he seems fine then stay without one.
The last thing is to make sure that there is no way for your snapper to get out of its take. They are suprisingly good climbers. My first snapper managed to climb his basking rock and climb the canister filter. I was lucky to find him there before he fell off.
Last thing, if you think you will be keeping your snapper long term, let it get used to being handled. Snappers are wild animals and each has its own personality but most can be tamed. I have no fear of my snapper biting me when I have my hands in his tank or when handling him.
Good Luck.
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Mike
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2 Painted
1 Musk
1 Common Snapper
1 Cooter
1 Map
1 RES
1 Corn Snake
1.1 Water Dragons
2 African Frogs
1 Toad
1.0 Degu
1.2 Chihuahuas
0.1 Cat