Hi Patrick,
Green water is caused by too many nutrients in the water and/or
too much light. Changing water can often make things worse because it
can contain large amounts of nitrates and phosphates and that's just a
free lunch for algae. The food you give the fish gets turned into plant
food eventually and your lava rock will turn the ammonia the fish
breathe out into nitrate as well.

Is the bag of lava rock placed such that the water is forced to go
through it or can it go round? It should be forced to go through the
rock and ammonia is converted by bacteria to nitrite and then nitrate.
Look up the ammonia cycle if you want more info.

You need to get rid of the nitrate to get rid of the algae, or get rid
of the light, and the best way is plants and more plants to use it up.
A filter with a UV light in will get rid of it as well. You can't put
just a UV light in since it will kill the algae but it will just rot
down and turn into more algae food, the dead algae really needs to come
out.

Is there any way you can build an area where you can place loads of
plants and have the pond water run through it? That would be the best
solution. Nitrate is toxic to fish in high concentrations and algae is
often a sign that nitrate is high.

--
Regards - Rodney Pont
The from address exists but is mostly dumped,
please send any emails to the address below
e-mail rpont (at) gmail (dot) com