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Since three years I am growing chili's inside my apartment. And every year I have huge problems with fungus gnats (Hope thats the right translation). I replace everything at the start of a season: -> new chili plants and new soil. The only thing that stays the same is the fertilizer I use (a bio tomato fertilizer).
So I started to wonder: Is it possible that the fertilizer I use is the source of my pests problem ?
Your tomato fertilizer is not the obvious contender as a cause of fungus gnats. The most likely explanation is the soil you're using in the pot - if it contains decaying materials, or its non sterile soil, or isn't a proper commercially produced potting compost, and the soil is constantly moist, that would explain it.
Let the soil dry out somewhat between waterings - when you replant next time, wash and sterilize the pots you're using thoroughly, then fill with new, proper potting compost, and don't overwater. Actions to take now to reduce the current problem here http://homeguides.sfgate.com/kill-fungus-gnats-houseplants-dish-soap-87288.html
