Question
Hi, I done a 20% water change from my 90 litre tank, which
contains an 8" pleco catfish, 6 female and 3 male Guppies, 2
female and 4 male Cherry barbs, 4 haerlequins, 5 Clouds, 3
penguin tetras, 3 neon tetras, and 2 small clown loaches.

The Female Guppies were all ready pregnant when I got them
and are fairly new to the tank but have settled in fine, and
1 has even given birth, after the water change I noticed my
harlequins fins and tail were red and they seem to be
spreading their dorsal fin alot more than usual,ive tested
the water with the API master kit, the results were nitrite-
oppm/ amonia-oppm/ ph-6.6--6.8/ high ph-7.4.

is it stress or is it because mu penguin tetras are territorial is there anyway I can help the harlequins, Ive
heard of Aquarium salt, but im not sure if it will work?
thanks for all your help.  

Answer
You don't want to add salt to your aquarium.  Check for nitrate and general hardness.  High levels of either may contribute to red fins.  The direct cause of the red fins is a mild bacterial infection.  If the problem persists, consider getting rid of the pleco because it creates the most nitric compounds of all your fish.