Red Tailed Shark Poorly

[purple haired dolphin]

hi, I've got a red tailed shark, a few thousand (well, ok, 30 ) guppies and a loach. Last week I transferred them all to a new, bigger tank and all seemed well.
I used as much water as I could from the old tank and added de-chlorinated water to top it up, used the filter sponges for about 4 days but had to take them out after that.
Last night I noticed that the RTS was resting on the bottom (not unusual, it does that quite a lot) but his tail and body seemed extremely pale. His tail was almost pink instead of the vibrant red that it usually is.
I added Pimafix to the water as it seemed like the only med I had in that was anywhere near the right thing, not knowing what's wrong. His tail and body look a lot brighter today, but he's still much stiller than usual and he certainly isn't himself.
I'd have thought if it was the stress of being moved it would have affected him much sooner that this, he seemed fine at first. All the other fish seem absolutely fine.

Has anyone any ideas as to what it might be and how I can treat him/her. it's about a year old now.

Thanks

 

[Thomas 2507]

What's your water parameters like?

 

[purple haired dolphin]

I've not had chance to test it yet. In the old tank they were fine and it's the same water, apart from about 20litres which I added after treating

 

[WILDER]

Don't add meds still you know what wrong with the fish.
How many gallons or litres is the tank.
How many fish and which type.
Water stats in ammonia, nitrite, nitrate, and ph.
Being pale can be many things so need to rule bad water quaity out first.

 

[purple haired dolphin]

128 litres
2 adult male guppies
about 25-30 young adult-baby guppies
3 cories
1 loach
1 RTS

New tank but old water from old tank (100 litre)
No new fish added
all were happy and healhy in old tank.

I can't check the water stats till later

 

[WILDER]

Need to know water stats as being pale can be bad water quality.
Red tailed shark really need 40 gallon tanks.