Checked my butt's contents earlier and it looks like soup with a green film/crust ![Sad :(](https://chat.allotment-garden.org/Smileys/green/sad.gif)
How long is the collected water viable before it becomes an actual hazard to plants or of no benefit?
Two things can help. Make a filter for the downpipe to keep out any particles. I made one from a milk container turned upside down, with a hole cut in the bottom to fit the pipe. Fit a length of pipe to what was the top of the container to support the filter. The pipe rests on the bottom of the butt. Cut some coarse slots in the bottom of the pipe, or drill some large holes near the bottom.
You will need to cut a flap in the side of the milk container so you can put some coarse pieces charcoal to support a piece of coarse sponge. On top of the sponge put a layer of finer charcoal, and a fine sponge on top of that.
The filter keeps out the particles, and the pipe from the filter to the bottom of the tank means that new water goes to the bottom of the tank, so provides oxygen and circulation to the bottom.
The other thing is to make sure the butt is light proof and keep it as cool as possible.
AFAIK you only need to throw the water out when it smells of rotten eggs. Caused by getting crud in the bottom of the butt.
Greenhouse seedlings are suffering enough with greening compost tops - surely watering with this will only make it worse?
Fine charcoal about 1/4" deep round the seedlings will keep the top of trays clean.
We completely stopped Basil damping off by doing this,
and it was stood continuously in a water tray. Again with charcoal in the tray.
You will need to get British made charcoal, as most imported product is too dense.