Barrel pond gone stagnant

I recently purchased half an old barrel to create a pond. The barrel is
about 90cm in diameter. I originally stocked it with a few plants, some
oxygenating plants and added a small fountain.

However, within a few weeks the water started to smell. I also noticed
a number of small grubs in the water - some of which have grown to the
size of maggots. The water also has the oily surface that you get on
stagnant water.

I have drained the barrel and started all over again!

Does anyone know what I might have done wrong and how to avoid this
next time round? Also, are the plants OK to use again? Their soil seems
to contain a lot of the white grubs.

Thanks.
Colin E.