There is no need for a larger tank if snails and crabs are living in this one. There are just things that need to be improved on in this tank, or you'll continue to have the same issues even with a larger tank. Yes, a larger tank will provide a bigger margin for error, but I don't believe that it's your water chemistry, so a larger tank isn't going to matter.
Hexagons make for terrible saltwater tanks. Corner tanks can be tricky for proper water flow/circulation in saltwater reefs.
How do you acclimate your fish?
IMO, I would do a 50% water change, or as large as you can do.
It does not look like you have that much live rock in there or much surface agitation. IMO, Koralia Nano's just plain suck. I have one my 14G and it barely moves the water around. I would get some live rock. You could go the route of uncured, which would mean recycling the tank, which at this point in time might not be a bad idea. This will kill the snails and hermits, so you might want to see if you can trade them in to your LFS.
What kind of sand is in there?
The hydrometer MAY be off. Is it a swing arm or floating model?
To me, my guess is that the tank never fully cycled and is fighting to play catch up. That coupled with poor oxygenation of the water and flow issues may be the problem.
Do you, or have you ever ran carbon?
Another thought that I'm going to throw out there:
Stray voltage? how do the fish act before they die?
The food you were feeding:
Flake food is garbage. Feed better foods such as Mysis shrimp, Emerald Entree and Marine Cuisine, any GOOD store is going to stock those items and they are available online as well. Flake food is like candy to fish, it does not have much nutritional value and really has no place in the saltwater hobby.
The fish:
Did they all come from the same store? This alone could be the issue.
Chromis do better in groups of 3 or more. When they're kept as a pair once usually kills the other. This is especially true in smaller tanks, they will kill each other. IMO, Chromis and Clowns will tend to be aggressive towards one another in a smaller tank as well. This aggression can become more of an issue if there aren't proper hiding places for the fish (rock).
A good stock list for you tank (and this can be played with):
2 Ocellaris clownfish
1 small goby (a yellow watchmen or Yasha Goby or something similar in size [3" or less])
1 Tailspot Blenny or BiColor Blenny (a tailspot is better suited for your size tank)
1 other SMALL fish such as a firefish, Orchid Dottyback, etc might be okay later down the road if parameters are in check.
5 Nassarius snails
5 Cerith Snails
10 Trochus snails
2 Skunk Cleaner shrimp or 1 Fire Shrimp
Ditch the hermits - in small tanks they'll kill the snails just for sport
15lbs of live rock
10-20lbs of live sand (I prefer20lbs in my 14G).
