I'm not very familiar with SW aquaria, but similar rules apply. If you had an "ammonia spike" last week, this week you're going to have a problem with nitrite. That's the next step in the cycling process. The chemical you were given was probably Ammolock, which whilst it removes ammonia, does diddly squat for nitrite. Nitrite is highly toxic to aquatic life - inhibits their ability to process oxygen, and takes much longer to be broken down into more harmless nitrate (the final stage in the cycling process).
Ammo-lock wasn't a cure, just a temporary relief. What you actually needed was for the beneficial bacteria in your tank to grow to the point where they could break down the ammonia immediately into nitrite, and the nitrite into nitrate.
You need to get yourself a nitrite testing kit and you need to test for nitrite every day. You also need to do a 10-15% water change every day, twice a day if the nitrite levels are extremely high. It'll probably take a few weeks before you can relax, but once your tank is cycled it should be relatively plain sailing. Sadly, your crab may not make it, and your damselfish will be lucky to survive.
To speed up the process, try to get some matured filter media and some sand or gravel from a matured tank (ask your LFS if they can let you have some). That will contain the bacteria you need to get the nitrogen cycle going in your tank. Put the filter media/gravel in a stocking and suspend it in your tank, or if you can, stuff it in your filter.
For future reference, most people recommend a process called Fishless Cycling for a new tank. Instead of risking living creatures, you provide an artificial source of ammonia for the beneficial bacteria and you don't add any fish or creatures until both your ammonia and nitrite levels are zero.
Good luck!
brown algea(diatom) is a natural part of the SW cycle.It should go away and a nice green should start if all else is proper.......Air
