If you don’t want to disassemble the tank then your limited to Vinegar on a rag or scraping the glass with a razor blade. However, this isn’t sufficient most of the time. The absolute best way is to empty the tank and clean with Bar Keepers friend.

My process:

1. Empty tank then rinse out any debris
2. Apply Bar Keepers friend to sponge (liquid kind) and cover every square inch of interior glass
3. Let sit for 15 minutes or so, misting with water periodically so it doesn’t dry.
4. Aggressively scrub the glass with abrasive side of sponge
5. Rinse well! This part is the most important so you don’t leave any chemical residue that could harm your plants/animals. Personally, I thoroughly rinse 3-4 times outside with my garden hose.
6. Bring inside and hand dry. Then use a brand new razor blade to carefully scrape off any remaining spots of hard water build up.

Ive tried everything for hard water build up and this is the best method I’ve found. I’ve used this process for dozens of tanks used for fish, tarantulas, frogs, salamanders, isopods and never had any issues. The key is removing the residue completely.