I think that's the bottom line. You may have added fish before you were cycled. Here is what you want to achieve:
1. no ammonia at all, even if you drop in a little fishfood. It disappears, and there's still no ammonia. That means the tank can handle waste.
2. temperature 79, day and night
3. salinity 1.024. Constantly, no variations. Draw a line on the tank and keep it filled to there, testing salinity daily until you figure how it works.
4. nitrate under 20.
5. alkalinity between 7.9 and 8.3.
Once you're there, then you can add crabs and snails for four weeks until the tank is fully ready. We'll walk you through this, but these are the points you have to hit before you consider fish. You also need a quarantine tank and separate pump for that.