How to seal a terracotta pot as a fish friendly pond?

Would anyone have experience of sealing the inside surface of a large terracotta pot so that it can retain water and becomes a small fish pond?
The trick is to use something environmental and fish friendly.
Google doesn't seem to provide an instant answer?

How big is this flower pot, and where is it to go?

Since it has been over a week and PDMpond has not replied, I thought in case anyone is wondering why I asked the size of the pot, it is for fish welfare.
The average flowerpot is way too small even for just one fish. Each fish requires roughly 5 litres for every 2.5cm of length, not to mention flowerpots are known to split in winter.