Fertiliser
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Identification
Giving a cactus too much fertiliser (especially nitrogen) will result in weak growth which is susceptible to pest and disease attack. It is better to grow them slowly, rather than "force" them.
Over-fed plants might also not flower as well.
Prevention
Feed less than any other houseplant, even once a month in summer is ok.
This echinopsis is underfed - feeding needs to be increased until the young (yellow) plant is as green as the centre of the mother plant to the left of it :