Pond sealer & dead plants

A few months ago we applied a 'pond sealer' as our concrete based pond had
sprung a leak.
Since then all has been fine with the fish & no leaking. They have even
spawned & we have young goldfish (fanatails & shebunkins) thriving in a
seperate area.
All the plants that we had previously were vigorous & needed constant
cutting back but have now died.
I went to a bit of expense & got new ones - same thing happened.
I could see no warning on the can & we followed directions to the letter re
changing the water etc., before re-introducing the fish again.
Is there anything we can do as we live in a hot climate & the fish need
protection from the heat. We do have shadecloth over the pond but would also
like some plants to survive.
Anyone got any ideas please?

Ta Pat