Hi, there, and thanks for joining the group!
I know some others will chime in, but I wanted to throw a couple things out there for you to get started on.
You want your substrate to be six inches deep minimum, deeper is always better. Crabs love to dig, and like to go deep when they molt. You can either stick with the eco earth, or add in some regular playsand.
You want to add a hygrometer as soon a possible. It is critical for crabs to have a minimum of 75% humidity; less can damage their gills. Higher is also fine, just watch for mold it you keep it super high.
Bottled salt water for crabs, in addition to being expensive, really isn't all that great for them. It's not harmful, but it doesn't have everything they need for long term health. What you really want is two things, Seachem Prime, for dechlorinating (or another dechlorinator that removes/treats chlorine, chloramine, and ammonia) and Instant Ocean. Both are available at pet stores. With those two products, you can safely use regular tap water, and your saltwater will be much more beneficial. Also, you'll find that it will end up being a lot less expenive; the smallest packages of each will last you a long time.
Eliminate the crab pellets, and feed from your kitchen. Many of the foods we eat are fine for them, as long as they are not seasoned. Check out the safe foods list, you'll get lots of ideas.
With six crabs, you'll want to make lots more shells available. Figure on 3-4 per crab, at least. JoAnn's and Hobby Lobby can be good places to find shells, as well as dollar stores sometimes.
Sponges are not neccesary, and unless you sanitize them daily (not just rinse) they can grow bacteria, and not the kind we want in our crabitats. Most people think they are simply not worth the effort required to keep them safe.
Maybe most importantly, don't get overwhelmed! There's sooo much information to learn, and most of us have had that 'I'm not sure I can do all this' moment. Just take it one step at a time, and remember we are here to help, and are more than happy to.
This is skeet