If the black things wriggle and have a tendancy to float to the top then they are Mosquito larvae. Put some washing up liquid in the water. This will break the miniscus and they'll drown as they use the surface tension to support themselves.

For the green you could put in antialgas - really meant for swimming pools. This is an algicide and should help to kill off and keep the algae at bay.

I assume you have one of the floating dispensers for your chlorine. These do help to keep the water clear but the tablets do dissolve very slowly.

Your best option will be to use up all your water and then get into the balsa and give it a thorough clean out with bleach - if it is fed by rainwater then it'll probably have an accumulation of decaying vegetation in it that will quickly turn the water green.

Nigel

You say the balsa is covered, but how well? Algae needs light, as does pondlife. Don't panic too much about the wrigglers, they are as likely to be midges or gnats.

I suspect the requirement of a filter when using algicide is to filter out the algae. It is possible to buy small circulation pumps and filters, but I don't know where.

If there is a lot of muck etc. suspended in the water you might want to try liming it. You can buy a bag of cal from the builders merchants - it is large white crystals - not the stuff that looks like cement.  see: about lime.... But remember to wear eye protection and gloves - this stuff will boil water and a speck of it in your eye will boil your eye.

I agree, though, with Nigel that you should (eventually) drain and thoroughly clean the balsa before refilling it. It is possible, and cheap, to dose with household bleach, but you have to buy the one that says it's suitable for sterilizing water - if it says on the label somewhere "no apta para la desinfección de agua de bebida" then it's the wrong one (translated: not appropriate for disinfection of drinking water). I think it just means it's perfumed, so look for an unperfumed one. As for the dose, if you want to go this route, let me know and I'll dig out the gen. Certainly, you will have much better control of the chlorine content this way. Swimming pool levels are too high, really.

As a footnote, you don't know for sure where your water is coming from or what other source residues there are in the tanker, so it might already be heavily loaded with incipient algae and pondlife. The guys that deliver water mostly deliver to farms where water quality is not an issue.

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