Very cool!
Is it a really young crab? I can't tell size from the pictures. Molting every other week sounds like there's a contaminant in the water - unless it's a fairly young crab. Did you ever allow any tap water to get into your tank?
I'm not a shark expert so take this with a grain of salt.
Multiple studies I've been a part of show that skimmers do not effectively remove Iodine. Algae of all types do remove it though. My suggestion to prevent hypothyroidism in your shark is to feed wild caught shrimp shell on. Like I...
I agree that slower is better.
If the Clownfish are a bonded pair then they will need to go in together. If you're just getting two random Clownfish then it's best to add one at a time knowing that the first one you add will likely become the dominant Clownfish and turn female.
This fits in with my guess as to what's happening.
Alkalinity is already high and then when you raise it with the "Buffer" it gets higher than what can be sustained and a reaction between Carbonate and Calcium occurs. The ironic thing is that this reaction often lowers PH back to where it was...
Only use "buffer" to raise PH if your Alkalinity is low. That is the ONLY time it is appropriate in a display tank.
PH should be treated as a symptom, not something you can directly fix. This of low PH as a Flu. Sure you get a fever and you can reduce the fever with medicine, but reducing...
The only question asked was "what's going on"....
There is no way for me to offer suggestions when the only water parameter provided is PH. I need all of them.
Sea Monkeys (Artemia, AKA Brine Shrimp) only produce eggs when they are stressed. The eggs will not hatch until the stress that killed the adults is removed. It could be water quality, temperature or lack of food.
They are omnivores with a preference for algae. They also like broccoli, carrots and stuff like that too but have no problem eating Mysis or Silversides. Very effective against Bubble Algae...
I had a similar (and smaller) Siganus virgatus. The venom is similar to a Honeybee sting.
It's not going to attack, you'll only get stung if you move your hand toward it when it's cornered and frightened.
I hand fed mine often without incident.