Fertilizing has been a topic of debate among Nepenthes enthusiasts for some time. I remember reading and hearing that fertilizing Nepenthes will cause plants to cease making pitchers. The theory being that Nepenthes are carnivorous plants by nature, so if they can obtain their nutrients through fertilizers the need to produce pitchers would not be necessary. 

As with any anything that is consumed, moderation is generally the key. I do fertilize my Nepenthes but not at full strength. I use a seaweed extract water soluble fertilizer like Max-Sea or Sea Grow. These fertilizers are a 16-16-16 base and can be purchased on the Internet or possibly at your local nursery. From what I've gleaned from the instructions, periodic fertilizing at 1 teaspoon / gallon twice a month or about every 15 days has proven beneficial. One brand recommends a full strength of 1 tablespoon while the other brand uses 1 teaspoon as full strength. So I've been going with the 1 teaspoon / gallon that works out to about 1/3 strength. There are other growers that use different orchid fertilizers at different strengths. The possibilities are endless and it seems that every grower is looking for that "edge" to produce the best results from their Nepenthes. 

The results so far have been very encouraging. My plants have improved foliar growth and pitcher production. Some older Nepenthes that desperately need to be repotted and living in poor soil have taken off again. Even the aging peat moss is turning green as well. The only negative is that the weeds grow incredibly fast with the use of fertilizers. The weeds can literally consume a pot within a few weeks but the trade-off is well worth it. I don't like pulling grasses and weeds out of the pots, but the increased growth of my plants is well worth the extra nuisance. 

With any advice, what works for me or other growers may or may not be the best for you. I grow my Nepenthes in less than favorable conditions. Indoor terrarium growers' plants grow differently than greenhouse growers' plants too. Experimentation and "tweaking" with fertilization are the key to successful cultivation. I recommend dropping in on the Nepenthes forums or asking growers that you buy your Nepenthes from their techniques and / or advice when it comes to fertilization if necessary.