I experimented with pouring acrylics (with flow improver, and a little silicone oil, sold for oiling treadmills). I wasn't quite sure how this would work, and I used too much flow improver so the paint got very runny and also I don't think I stirred the paint as much as I should have done so I got bits that were still a bit more solid that made flecks. It took a long time to dry. Possibly I should have bought some proper pouring medium (only that's expensive and I wasn't sure how much I'd use it) or perhaps tried PVA glue, which is supposed to be quite good and cheap. Still, the effect is interesting and it didn't all go to sludge. There were a couple of twirly marks that looked as though they were beginning to be mermaids, so once I'd got it to dry, I overpainted in a more conventional manner with a brush to make them into mermaids.
I learned that my ultramarine blue and yellow ochre paints seem to be heavier than the phthalo green,so when diluted, the phthalo green tends to rise to the top. Unless that was just an effect of having diluted the quanties differently, I didn't measure them.
Next time, instead of mixing colours in a cup, I think I shall pour each one onto the canvas separately so I have control of roughly which areas are dark and light. I ended up pouring on extra paint with this one, because it had all gone a bit too uniformly green, probably because I diluted the paint too much. I added a streak of bright orange, the dark blue on the left, and some random blobs of pale yellow which seem to have entirely transmuted into other colours.