Diseased Tank?

Ok so I bought some new rainbow's the other day for my tank and a pair of endlers. (My tank is almost 50 L long and now has 5 rainbows, a pair of endlers, 3 kulli loaches, pecoltia and some amano shrimp and is heavily planted with a collection of hollow logs and a homemade coconut house).

One of the rainbows developed a whit covered eye within a week and was dead the next day. This all seemed to develop rather quickly. I added some melafix to the tank in the hope it would help, but I guess it was already to far gone.

I thought she may have grazed it on a branch and it gotten infected since it would've been a new tank and it's quite heavily planted. The way they dash about it can easily imagine it head butting it by accident. However my little kulli loach is looking poorly now. The gills seem inflamed and it's colours seem washed out. It also seems to not be very active and boisterous. With more of a random squiggle than a purposeful wriggle. 

All my other fish seem fine and active.
I tested my waterstats :
NH3/NH4 0-0.25
No2: 0
No3: 0-0.5
pH: ~7.6

So my pH is a little high, but not excessively for our region, and it doesn't seem to be caused by an imbalance in the other stats.

So what is the most likely cause? Disease; he's not eating enough since he's last in the pecking order; one of the other fish is bullying him or maybe the melafix?

I've had him for almost 3 years now and I'd hate to give up on the little guys without a fight!

No it's not a new tank. There are new fish. How can it be a new tank if I've had the loach for 3 years?   

Yes it's well cycled and has been at this location for over a year. I say this location, since I had to move house last september. The move is the reason the tank lost some fish. (Had to travel across the UK, long long drive with a trailer). Had only 2 rainbows left, next to the pecoltia and loaches. The shrimp were added around february. This 'tank' was set up 3 years ago though.

The dwarf rainbows and endlers are new additions though (since having 2 dwarf rainbows wasn't fair on them). I have a double filter system. (One is air powered and one a regular suck water through one, forgot the brand). I checked my waterstats every 2 weeks, but did them every other day to keep an eye on the tank with the new additions. So I know it's not the water quality. The temperature hasn't changed either. 

The loach was fine with the dwarf rainbows before. No issues regarding aggression, but with the addition of the females (the ones left were male) perhaps their behaviour has changed?