Pilot flying approach reported low altitude alert from ATC during approach in IMC.

Narrative

Approach gave me a descent to 2500 ft. in preparation for the RNAV XX into ZZZ (Night IFR). The aircraft was flying under autopilot at the time at a slow descent rate (-600 fpm) and does not have an altitude arm mode. I did not notice I had passed my assigned altitude of 2500 until I was as low as 2000 MSL. I immediately disconnected autopilot and performed an escape maneuver back to 2500. Simultaneously; approach contacted me about my altitude and requested I climb immediately. I was back at 2500 ft within 20 seconds.This was caused by me misreading my altimeter during my instrument scans; and seeing 1000 ft higher than I actually was (analog altimeter). That being said; me noticing in time; approach; and the eventual terrain warning I would have gotten had I continued descending on both the GTN650 and Foreflight are all very good last resort safety measure that would have allowed me to recover from the situation had it gotten worse.500 ft. deviation on any IFR flight is not acceptable given a potential Controlled Flight Into Terrain result. I will be putting significantly more time practicing scanning flows in flight as well as in simulators.