Nitrate level has spiked! Help!?

I've been out of town for a week and before I left I did my regular weekly water change (about 30%.) The tank of 1.5 feet by 0.5 feet by 0.5 feet. As soon as got home, I checked on all my fish (male guppies and endlers) and they were fine, so I was hoping I didn't have much to worry about...until I tested the water...my nitrite level and ammonia level came up 0ppm and 0.25ppm respectively, but my nitrate level was mive! almost 80ppm! I've since done two more water changes, once a day (about 30% each time,) but my nitrate level is still above 40ppm and the nitrite and ammonia levels still have not budged...Anyone have any advice? My fish are all still fine, but I still really want to get my nitrate level to someting more accpetable.