Reducing size of pond - old swimming pool

[Netmask56]

This is a bit of a brain pick! I converted my old swimming pool into a pond with fish 5 years ago. It's 8m x 2m by 1.2m at the shallow end and nearly 2m at the deep end. So it contains about 30,000 litres and about 30 goldfish who are doing well. I would like to displace about a third to a half of the depth with some lightweight solid mass that won't float of course but won't add too much weight to the concrete pool itself. Any thoughts on materials I could use? Obviously bricks and stones could impair the structural strength of the concrete pool. So something like polystyrene but unlike polystyrene will sink to the bottom.

I suspect there will be water shortages in the new year in Sydney Australia as we have just come through the driest and hottest winter and autumn for all time, so I would like to reduce the amount of water I use in order to keep my rainwater tank for the garden.... A friend suggested volcanic rock! A good idea but would cost an arm and leg or two for the sheer volume I need. Any thoughts?

 

[j.w]

Well if you can't find anything that will work you could take the water out and fill the pool w/as much dirt as you like and then put a liner in there. Someone else on here is doing something like that. I found it: https://www.gardenpondforum.com/threads/swimming-pool-into-a-pond.22196/#post-357761

 

[GBBUDD]

There is a product that any concrete company can make for you it's called flow fill in the north east. it's substantially cheaper then concrete and it's virtually self leveling. the fish should be removed from the pond/pool and you would want to pour the flow fill through what is a basically a large straw that is called a tremie pipe. the idea is to pour to the bottom of the pool using this large straw/ concrete pump or tremie pipe. once the initial flow fill is in and at the bottom of the water you pour the rest into the previously placed flowfill never removing the / straw from the concrete/ flow fill thus you don't water down the concrete you pour the new into the already placed and if it was me I wouldn't trowel the concrete in any way i'd go for a uneven bottom it will look more natural. it's a proven method for pouring concrete trough water and where you don't need structural strength flow fill is a perfect choice, it does not harden as hard as concrete you can scrape it using a planters shovel.