On a warm windy day it is likely that you will lose 2 inches of water from your fish pond to evaporation. However, a leak should not be discounted as the cause, and the added precaution of sealing the inside surface of the pond isn't a bad idea either.
First, get your fish into safe temporary quarters, completely drain the pond and let it dry out completely. Drying makes algae easier to remove. Use a wire brush for this job. Chemicals are not a good idea. They may lodge in the concrete and be a danger to your fish.
The cleaning will expose hairline cracks if any exist. These should be patched. Quikrete makes a super line of concrete patch products. Follow the instructions on the box concerning mixing and application very carefully. Use any latex base concrete sealant (insure the one you select does not contain mercury or other anti-fungal chemicals) rather than an epoxy resin. Although the epoxy based sealer would work better to prevent leaks it may be harmful to your fish. Once the job is complete refill the pond, put the fish back in, barbecue some hot dogs and make sure we get an invitation.