How to clean all of the weed out of the 4ft tank
How I did it: So we did it...by emptying all of the water out of it!
We did try multiple doses of algaecide, some of which seemed completely sucessful. It kept creeping back though.
We're moving in a few weeks, so the tank had to be emptied out in any case. We're going to thoroughly clean everything that was in the tank. Once we've moved, I'll let all the lovely little pebbles and such soak up some sunshine for a couple of days. Once I'm happy with the state of everything, we'll set up the tank again and see how it goes. If it seems clear after a couple of weeks, I'll start adding plants and go from there.
Lessons & tips: Get on top of it what you first see the stuff growing. Don't let it take over the tank - its far too difficult to stop once it gets a foothold.
Resources: http://www.tropicalfishforums.co.uk/
An immensely helpful forum - I'd recommend it to anyone who has or is interested in tropical fish.
