What is happening to my puffers?

Yesterday when I came home I noticed one of my puffers had died. I was away for the week and had my son feeding them , from premade packages so it wasnt overfeeding. My test results seemed normal except for the high GH, and lowing of the PH.
I added yesterday 4 scats, 3 mono sebeas, and 3 mono argents before I realized he had died. Today I noticed The other dwarf puffer has darkening of the belly which is usually a creamy white.
Any one have any ideas.

After a water change my results were

KH stays steady at 7 or 8 drops = 70-80 mg/L,
GH 960 mg/L which is steadly rising
Ph is 7.6 slowly going down (was at 8 last weekend)
ammo=0
Nitites=<.03
Nitrates= 10~20

Tank is a 55 gal.
Unfortunatly these puffers were given to me buy a friend who was told they were figure eights. I then from posting pics was told they were green spotted puffers, Then after going down to the store where he got them, 3 hours away. The store had them listed as dwarf puffers, but another store in the area said no they are actually green spoted puffers.
This is the same tank as the other post I asked about high GH.

They have been inthe tank for a little over 3 weeks. They are only aprox 1 inch nose to tail.
Unfortunately I have been away during the last 2 weeks but home on weekends due to work. Every weekend I do at least 20% water change. And they were in the tank with 3 columbian sharks, sailfins and a molly. The new additions were only yesterday.

My QT tank has snails and guppie feeders in it with no salt, should I move him there or is he maybe beeing stressed from the new additions. being that I havent been around to see them daily, I am at a loss.

Thanks Goldfish Freak.
I hope that is what it is. The one that died didnt have any grey post mortem. I am not going away again for a while so I can keep an eye on this one.

Water quality is not too much of a problem with me, at least once a week if not more water changes and I am always spending tooooo much time with the fishies says my wife.
Larger tank comes probably this summer, as we will hopefully be moving to a new posting. As long as I dont have to go away for any extended periods things should be good. I also have another 30 gal sitting in basement which i plan to start up with tank water from the 55 if need be.

I am planning on getting a protein skimmer for the 55 cause eventually I will go marine. I read somewere that you can use it in brakish water? RTR, what are your thoughts on this ?
Also I have read to raise PH use baking soda? How much would be used ?