I just ordered a cheap PH meter. £15 inc delivery. I will hold fire on the EC/TDS meter. The nutrients I will pick up locally for £11. The air pump was £15 The trough, net cups and clay pebbles which I had already perhaps a tenner. This watercress had better be productive. Hopefully next year I will be ready to have a larger unit outdoors in and out of the greenhouse. In retirement I will be setting up a full-scale hydroponic / aquaponic facility. Meanwhile I am experimenting with vaseline mixed with salt smeared on the window to discourage my snaily friends from entering and grabbing my watercress before I even manage to harvest any.