Im planning to make an wooden indoor pond with the dimensions of 15x4x4.5 ( length/depth/width). What should be the thickness of the would should i use?thanks in advance
With that size you're going to need to brace at regular intervals with either 2x4's or 2x6's (probably 2x4's should be fine). You'll want to use at least 3/4 hardwood plywood. For extra strength, you could double it up, but with proper exterior bracing, that shouldn't bee necessary.
As far as "tarp", I would try to find actual rubber pond liner. They generally come in 20mil and 45mil thicknesses. Thicker is better. If the pond is going to be outside exposed to sunlight, then you'll want to go with a thicker UV resistant liner. Firestone EPDM 45mil liner is pretty much the standard for fish ponds.