Over the past week I have been perplexed by what appears to be an algae infestation of my own design, as I hadn’t taken any steps to keep light our of either the reservoir or the upturned open ends of the garden beds.
In an attempt to both starve out the colony that has already taken hold, and prevent a new one from being able to set hold in future plantings, , I’ve pulled a couple of plastic lids out of my recycling which I have found sit nicely on the elbow as end caps. I drilled a couple of holes in one of the lids to run my irrigation lines through, and was able to get them in a better position than where they were previously without the lid for support.

This should take care of any light getting into the downspout.

As for the reservoir, I’m going simple and low tech.
I’ve got a black garbage bag that I am using as a tarp. That will easily prevent any light from getting in the jug, and is dead easy to open/move when I need access to the growing solution.