HCA: Hermit Crab Association

I have two Jumbo crabs, Hermie and Magnum. Hermie might be closer to just Extra Large, but Magnum is a Jumbo.
They reside in a 50 gallon Long tank. The substrate is coconut mixed with sand. Currently it is about 6 inches of substrate.
I just found the size chart for the first time and took the crabs out to measure. That is when I found Magnum is "officially" a jumbo. This led to seeing that according to the chart I need 12 inches of substrate, double what I currently have.
The tank is about 17 inches tall, so they will have pretty much no space for climbing or anything after that. I have read about toppers, but they are expensive and I am on a very tight budget. Does anyone have any suggestions?

I would just add as much as you’re comfortable with. Since there are only two crabs they should have enough space that they won’t go after each other. The fact that they haven’t already probably proves my point

Having a diet that allows them access to plenty of protein so they don’t feel the need to cannibalize would also help


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Ok. I will add more as I can because I am wondering if one of them did go after the other one last molt (she survived, amazingly). What I feed them does include a protein, but I will try to up that as well. Thank you.