Is there any chance you or a future owner would ever want to restore it back to being an 8-10 feet pool? If not I imagine it would be similar to dealing with an old cistern (which I happened to have researched lately) - drill & break up the bottom for drainage before filling in the hole. the walls could be left up for the pond (otherwise top 3 or 4 feet knocked down.) I cant imagine hiring someone with a jackhammer would cost anywhere near $8,000, that must be for complete removal and refilling.

If you don't want the pond that deep and dont want to mess with the current pool structure I would consider filling the pool with any material you can easily get, to get the bottom up where you want it, then drop in a liner on top. I would not have the gravel actually in the pond, but under the liner as it will collect lots and lots of nastyness.

Some materials I am thinking for filling would be sand (soft on the liner. Or a layer of gravel, then sand.

Again I'd put the liner on top of the fill.

@pcan "I'd put the liner on top of the fill." I agree.

I did this for a gunite pool awhile back. I felt the same way, 8' deep is a lot of water. I broke a 3' square hole in the deep end and another in the shallow end which is required by code in San Jose CA. I built a block wall to divide the pool in half. The shallow end as a pond and filled in the deep end, then a liner and pea gravel over the liner for plants.

I broke down the pool edge with an 80lb electric jackhammer which I rented. The edge of a gunite pool is 12'x12' with lots of rebar. An 80lb jackhammer was barely large enough, took a long time. A larger jackhammer would have been better. To remove all that concrete, the fill needed and proper tamping, $8K sounds about right.

I filled in with concrete from the edge and then sand from a sand blasting place which delivered for free. They have to land fill otherwise.

It worked well enough.

Filling in the pool with sand or gravel and then laying a liner is not a good idea. If/when water gets under the liner it has no place to go. If the gravel fills with water the liner will float up. Busting a hole in the bottom provides drainage. Another choice is to leave a gap between the pool and liner where you can install a length of 6" or better drain or PVC pipe down into the gravel. A pump can then be placed down inside the pipe to remove any accumulated water as needed.

Just filling the pool with sand or gravel will indeed 'get gross'. But so what? Lots of things are gross. A natural pond bottom is going to be gross too by most people's standards. It's only gross if you have dig in it.

If I did this again I would just use it as a 8-10' deep pond. It doesn't use any more water than a 3' deep pond with the same surface area as far as evaporation goes. It doesn't get dirtier. The water chemistry and temp will be more stable. It's just as easy to clean. Looking back I made a mistake filling in and doing all this craziness. The cost to fill with water vs to fill with gravel...way cheaper. Way easier.

I would vacuum a couple of times a year. A ran the pool as an 8' deep pond for about 4-5 years before doing the conversion. A lot of leaves accumulated and big chunks of muck started breaking loose and floating around. That was gross.