I've had my 15 gallon tank for a month and a half, and everything was going great until about a week ago. I tested the water and noticed the nitrite levels were getting a little unhealthy. I checked the filter and it was really dirty so I changed it. But the levels keep getting worse. I've tried water conditioners but nothing is working. So far only one fish has died and the others are still very active. Is there anything I can do to get the levels back to normal?

First off, welcome to the forum, and the hobby

Second off, welcome to the wonderful world of nitrogen cycle establishment via trial-by-fire.

The "gunk" in your filter was actually the nitrifying bacteria needed to process ammonia into nitrite, and this nitrite into nitrate. By swapping the filter out, you effectively removed the bulk of your tank's ability to process that nitrite.

If I were you, if that filter material is still in the garbage, swap it back in. Otherwise, the only quick fixes I can think of are immediate daily 50% water changes until your nitrite gets back to a manageable level, and/or a bottle of Tetra SafeStart to add some more bacteria into your tank. TSS is a great product but it in no way will cycle the tank completely. It will, however, alleviate your problems in this case. The water changes are VITAL if you want to save your fish though.

Sources, if you care to read a bit more on the nitrogen cycle:
The Aquarium Nitrogen Cycle
Fish Poop and You: A Primer

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Officially fishless until after the move....tune in later for details!

Unfortunatly I no longer have the old filter cartridge and I've tried 2 water changes in the past week. The levels aren't getting worse but they're not getting better either. I new changing the filter would change the levels a little but not to the point where its not getting better. But if daily water changes is what I'll have to do get it better I guess I'll do it. Thanks.

No problem. And remember - only do the water changes as needed to keep the nitrite concentrations down around 1-2ppm. More water changes than that might actually stall the cycle by removing the bacteria's food source (i.e. ALL of the nitrite).

__________________
Officially fishless until after the move....tune in later for details!

As recommended, water changes and patience. The bacteria that take care of nitrite seem to take longer to establish than those for ammonia. The good news is that if you keep the levels reasonable, after the cycle is complete the fish will eliminate the nitrite effect (methemoglobinemia) from their systems.