A plastic tray with a gravel layer intercepts the flow from the filter pump, channeling the water through the gravel where watercress is rooted. This way, waste from the goldfish becomes manure for the plants, and all are happy:
- the aquarium water is cleaned of nitrates and ammonia: the fish are happier and we don't need to change the water so often;
- the plants are fed and will provide us with a constant supply of fresh watercress.
This is just the very first experiment. I'd like to try growing edible fish instead of merely ornamental goldfish (you can just about see them in the picture) and scale it up: for all the optimism I doubt we'd get frequent harvests of watercress from such a small bed.
