Not entirely sure what you mean, but I think you mean the decaying of the cucumber is feeding/making the plants healthier. If so, I believe it is the cucumber creating ammonia-nitrite and that is feeding the plants. So I would assume the plants are not getting enough nutrients. You could add ferts (liquid or dry) to water column and/or root tabs into the substrate.
I don't think the decay has to be from organics as long as it is rotting and creating ammonia. So overfeeding would create ammonia (don't think fish food would exactly be considered organic). Other than decaying leaves and dead fish, I can't really think of other sources at the moment.