What is wrong with my fish?

Question
QUESTION: Hi, Ok I will try to be brief and still explain.  I am fairly new to the hobby. I have a 55 gal. with One big black molly 3 black skirts 1 glass fish 4 danios, 1 male and 4 female guppys 2 albino cory's and two snails. I do 25 to 30% water changes every week, adding the conditioner to the aquarium bucket before I add it to the tank, all of my tests are great, ammonia,nitrate,nitrite, and the tank is crystal clear.  All of the sudden I am losing a fish about every other day for no apparent reason.  My guppies used to play in the bubbles of the castle all the time and now for the last few days they don't play and really don't even hang out together.  I noticed it first with my black skirts and now think I see it in some others (if it's not just fear) that they are getting dark in color. Actually almost light black.  Please tell me what to do.  I love them all but I have my male guppy.  I call him yellow tail and if I lose him OUCH!! He even appears to be getting darker or not so brightly colored.  I am wondering if I should be feeding them the color enhancing foods? or maybe I am hoping that's all that is wrong.  I do not have a pet store around here that I trust or one that knows anything about the fish sadly to say but true.  Any ideas?  Thanks so much!! Beth

ANSWER: Define "great". What EXACTLY are your parameters.

Anyway, I can see right now a problem. Your black skirts should be in schools of 10, your danios in schools of 6, and your cories in schools of 8. However.....that shouldn't be killing your fish right away. That is likely what is leading to the whitening of the black skirts.

Have you noticed any signs of disease? Little white grains of sand on the skin? Reddening or exposed vains? White or black patches of skin? Clasped fins or protruding scales?

Also, you should be doing 25% or 30% water TWICE a week. That is likely what is stressing your fish out. Stress doesn't kill them, but it makes them weak and open to disease and infection.

Also, what is the temperature of your tank? Does it change temperature fast or often?

-Nick

---------- FOLLOW-UP ----------

QUESTION: Thanks Nick.  1st, I did have 4 black skirts and they are not whitening, they are getting dark. 2nd, I had 5 danios and the 4 left actually look ok.  I know I need to get more and was planning to but don't know if I should til I know what's wrong with them??  By Great I mean, I show little if ant nitrate, no nitrite and no ammonia and the ph is as it should be.  I will do more water changes and I keep the water temp between 75 and 78.  The only signs are the darkened color and like I said, my guppys don't seem to be having as much fun. Last night tho, it looked like some of them were like opening their mouths and closing them a lot.  Not really like breathing, more like seeing me standing there and so they were begging for a treat.haha They do that I am told.  Should I go ahead and get the fish I need to make the schools so they will be happy? or wait?  Thanks again Nick,  Beth Oh and no clamped fins and their eyes and mouths appear clear.

Answer
That is very strange.

The darkening in color is a good thing, it means they are getting used to the tank.

The only other thing it could be is some kind of substance leeching into the tank. Plastic decoration, colored gravel, copper medications are all things that can leech into the tank and slowly kill them fish, over time.

It could also be the temperature swings. Your temp should not be between 75-78. It should be 78. Not 77-78, or 78-79. 78. Period. Keep the temps stable. However, that shouldn't kill a fish, but it can stress them.

What are you feeding and how often?

-Nick