Guppies getting sick

Question
Hi Chris,
I had 4 male fancy guppies for several months who were doing really well. I went to the pet store, bought 2 more, and added them to the tank. At first they all seemed fine, then one morning (about 3 days after purchase) one of the new fish was dead in the corner. No signs of distress or illness before. The same thing happened 2 more times with my older guppies. There was no signs of illness, maybe lethargy, and I would just find them dead. So I have 3 left and am wondering what happened? The final 3 arent their perky, active selves and I'm worried they are going to die too. I have one guppy who is quite old and his tail is very frayed and looks like he's literally falling apart a bit. The other two look fine, but are mainly just hovering in the corner. As far as I can tell they are still eating. I have 6 neon tetras and 2 algae eaters that are doing great, no signs of any touble with them. I thought maybe the water from the pet store was infected so I did almost a total water change yesterday, but I still havent seen much improvement. One of the guppies does have a white patch on its tail, but it looks like just natural coloration, but I did buy medicine for Ich and added it to the water. If that doesnt work I really have no clue! I have test strips and the water is fine and the temp is in the high 70's. I am curious though what the best ph is. My ph is on the alkaline side. I dont know, I'm at a loss! Any thoughts would be great.
Thanks!
Jocelyn  

Answer
Hi Jocelyn;

Poor guys. Sometimes guppies have infections that don't seem to affect the other fish. Columnaris is one of them. Since there is a white patch on the one fish, that may be what you are dealing with. Sometimes there is no external sign of disease. The fish just die. Here is a good web page about it;

http://www.aquatic-hobbyist.com/profiles/disease/freshwater/columnaris.html

At Your Service;
Chris Robbins