The problem with household fertizers (and some older aquarium ones as well) is that they contain lots of nitrogen and phospherus, both of which occur in abundance in aquariums. Too much of these elements will encourage algae.
Yes, the aquarium specific fertilzers are overpriced, but you'll be less likely to be overrun by hair algae if you stick to them. (Not that their use will make you immune.) Generally speaking, aquarium plants only require additional potassium, a little iron and possibly some trace elements (although the letter are often replenished by water changes). Lighting, filter materials and other factors come into play, so there is no one "correct" formula for every situation. Depending on your water supply, you may not need supplemental fertilizer at all if your tank is well stocked with fish, although most planted setups benefit from additional iron.
