hi, sounds to me like it could be no good for fruit and veg, give us the name of the product and we can check for you, some products are for houseplants and contain metals that are not for human consumption and can make you very ill if ingested or inhaled, so be careful what you use,
NPK 8-10-12 is pretty much an all round feed but slightly more for flowering plants, not ideal for the plants we grow but if its all you got then it will do. so 5 tsp per cap, i give my plants 3tsp per gallon of water with the nutrients im using, so i wouldnt use anymore than 3tsp per gallon, i use a 2ltr pop bottle which is half a gallon and mix in 1.5 tea spoon per pop bottle, just try not to give to much, these plants require very little and its why nutrient companies producing nutrients for these plants charge higher prices, its because regular fruit and veg nutes are way to hot for these plants so they take the time to mix products to a good dose for the plants we grow,
so id start with a 2ltr pop bottle of water and add 1 tea spoon then feed the plants with that, then plain water the next feed, then nutes after that and see how it does, any chance you can post some pics so we can see how the plant looks then we can see if it needs more or less nutrients,