...it is very, very dark.
I'm doing the 'four day blackout' to get rid of algae. A bit of algae is natural and can even be decorative, but too much is ugly and stunts the growth of the plants.

The tank was starting to look a bit grotty, so I decided to take a water sample to a good fish store with knowledgeable staff and ask them to analyse it for me.
They did, and the water quality was overall pretty much great. But I was told that a good way to slow down the growth of algae (after hitting them with the Four Days) would be to lower the ph of the water, in other words to make it slightly more acidic.
A good way to do so, quite naturally and gradually, is to add peat to the filter. This will give the water a slight orange-brown hue, but the carbon that's also in the filter should take care of that.

So the tank is now covered in cardboard, that's supposed to shut out the light completely, and the lights don't come on either. It's a bit scary because I can't see what's going on in there. But the method calls for no peeking and no feeding.
I cleaned the filter and changed a bucket of water yesterday, fed the fish and Mike the frog well, and told them goodbye for four days.
What will the tank look like on Friday? No one knows.