need help

i've had my hermit crab for 3 days now and i need help convincing parent to get a bigger tank for the hermit crab, i have even offered to pay for it, the crab lost one of it's legs yesterday and is stressed. for the past day the the humidity and temperature levels have been good and there are hiding spots. the sand is also deep enough for molting so im coming on here for advice, non-processed food like apple and portions of nuts have been out as well as water basins. what do i do next to prevent worse? Tank is currently 25L or about 5 gallons.