HCA: Hermit Crab Association

Hey everyone! I switched our substrate in August from Cocofiber to sand because I was having issues with gnats. We have no gnats since but the tank smells. It smells almost like mildew but I’m not really sure that’s an accurate description. You can smell it from pretty far away.

I had the top of the tank covered with plastic to keep in the humidity and removed it, it didn’t help. I have taken out all natural/wood elements to see and it didn’t help.

I clean the tank, we do regular changes of the water and food, never more than 2 days. I even bought the Fulkers Prebiotic spray, it helps with the smell momentarily but comes back after a few minutes. These are my toddlers crabs gift from Grandma so I do all the maintenance and he’s learning but I’m about ready to rehome them because I feel I’m not doing something right and I think they would be better off with an experienced person but I would prefer to keep them.

2 purple pinchers
20 gal tank
Sand substrate
Fresh and mixes of food
Humidity around 75-80% when covered with plastic, uncovered drops to 62-65% but aim to keep it around 75%
Temp 72-75 - ¼ of tank has a heater

I'd suggest starting off with scraping the top 1/2 inch of sand away and tossing it, and wiping the glass and lid down with vinegar which is harmless to the crabs. Mold can grow on discarded food, and mildew sometimes forms on the glass. With no other decor it'll let you know if it is the actual sand mass or just incidental growth.

If that doesn't work and the crabs are up, I'd suggest turning over the sand and seeing if the smell if coming from the bottom of the tank. When sand gets oversaturated it can get compacted and cause anaerobic bacterial growth. Usually this smells like sewer or eggs, but sometimes it just smells weird.

A straight sand tank should smell maybe a little stale if its "healthy" and has no airflow. I've always thought it smelled like flowery rocks with a tiny hint of fermenting fruit from the crab waste. Mold or mildew smell isn't normal but can sometimes be unavoidable just because of the humidity and being sealed up. Boiling natural items in salt water can really help with preventing it though!