High Alkalinity, but no buffer use?

Hello all and thanks for reading!



I have been successfully running a 29 biocube for a couple months now. I had my water tested today and was shocked to find some levels have spiked. I have been gravel vacuuming and performing water changes weekly and I also just added a protein skimmer to my system. Just performed a small one a few days ago, but somehow ended up with this:



Ammonia: <0.25

Nitrate: 30 ppm

Phosphates: 1 ppm

Alkalinity: 15 dkh



I'm pretty confident that I'm to blame for the first three levels being high. I recently added a blue spot jawfish and have been overfeeding to ensure he is getting food. I spot feed him with a syringe but it's still very hard to get it to him, so I've probably been putting too much in there. I'm already planning on doing a 50% water change tonight and hoping that will fix the problem, but I am still confused by the high alkalinity. I have never added any buffers and the LFS didn't seem to have any clue what could've caused it other than that. Any ideas of the cause or a solution??