Sealant for concrete/brick pond

[Duckles]

The house I live in once belonged to my grandparents.  My grandfather was a brick mason, and in the 50s he built a brick/concrete/stone fish pond in the backyard.  Over the years it has had to be cleaned and re-sealed every 3-4 years.  The last time it was done was 2017, and it is leaking at an increased pace now, so I'm gonna have to do it again in the near future.

In the past, he used a white fiberglass-infused concrete.  It worked well for a while, but eventually would need to be redone.  I'm curious if there's something else you all would recommend for sealing this?

I've looked at Pond Armor, Herco H-55, and Liquid Rubber NeoPond sealant.  I had one local store tell me he'd probably just use Flex Seal.  Everyone suggests a liner, but because of the fountain in the middle and the irregular shape jutting out at one end, it would be almost impossible to use a vinyl liner without owning a tool to sonic weld pieces together to fit around, so I think I'm going to be stuck with some type of roll/spray on sealer.  By my math (Thanks, Fusion360), it's around 240sq ft of area that needs to be coated.

Have any of you used any of these products before, or what would you recommend.  

[MJV Aquatics]

I might also be temped to use Flex Seal, but the prep work for any fluid sealer to ensure good adhesion is gonna be a bear.

[Duckles]

The prep work to do concrete isn't exactly easy.  It's usually about 1 day of scrubbing, rinsing, and scooping with a shovel to get it clean enough that the concrete will adhere.  And the cure time on concrete is PROBABLY higher than any of these roll on sealants.  Usually the fish end up in a temporary holding pond for about a week to give it time to cure and be ready.

[gardenman]

Something that's used for higher-end pond liners is the polyurea spray on coating. (Like truck bed liners.) There are specialty firms that will come to your place with a sprayer and apply it. I saw a demo once where they stacked up concrete blocks with no mortar then sprayed them with the polyurea and they were about as strong as a fully mortared wall. If you can find someone locally who applied truck bed liners by spray, they could likely line your pond at a reasonable cost. The equipment is typically fairly portable. Google "spray on polyurea pond liner" for more information. And Flex-Seal should work also. The cure time for the polyurea is insanely fast. One pond builder used to claim they could fill and stock the pond within an hour of spraying.