I would suggest just two or three very small fish. I don't mean baby fish, I mean docile fish that stay small. Clown Gobies, Neon Gobies, Barnacle Blenny, etc.
Something doable that could be fun would be a Pistol Shrimp and some type of Shrimp Goby.
10 all the way. There isn't a lot of equipment acceptable for tanks smaller than 10 gallons in saltwater. I had a 5 gallon once, meh, it's just OK. Here's a picture of a 10 gallon I did some years back:
I've done nano tanks a few times successfully. The difficulty has already been discussed. If your goals are modest like you said then you should be able to pull it off.
We have a Nano forum here, I'd rcommend going through past posts.
How much live rock do you have?
For Ostracods (best food for Dragonets) some sand and what I call a pod pile can produce a whole lot of food. In a back corner pile some sand and surround it with reef rubble, small pieces of live rock, to help keep it in place.
Hi James and welcome.
We need to know a lot more about your system. Is this a new tank? How long as it been set up. How are you cycling it?
Listing water parameters is very helpful. It sounds like you have some ammonia in the water, have you tested the ammonia level?
Without knowing your Alkalinity values before the water change I can't know for sure if that was the problem. I'm going to guess it is. If so this will also affect the Clam but clams usually look great until they are almost gone. Are there fresh growth rings on the clam?
The supplement is a...
I don't know what animals you have. What I recommend is to pick your most sensitive animals, research where they have the densest population in the wild and then use something near the average temperature from that area.
Examples:
Catalina Goby - Population density in shallow water around...
Pick one as the master. Pick a temperature and try to be consistent with that temperature. Being consistent is more important than the actual temperature.
Green algae growth isn't going to negatively impact your fish. If you don't like the look of it then it's going to negatively impact the aesthetics of your tank. The algae will actually reduce Phosphates and Nitrates but unless you enjoy the look of flowing hair algae you'll need to deal with it.
The putty is fine for gluing big things. Just keep in mind it doesn't really stick things together well. It works mechanically by locking things together.