My guppies getting diseases

Question
I am trying to figure out why my fish keep getting
sick.  I have an estimated 12 fish in my 15 gallon
aquarium.  I have fancy guppies and an otocinclus.  Most of the guppies are small, they never grew to their full adult size.  I have 2 different species of those tiny snails too.  I have quite a few snails but it  doesn't seem like I have millions.  It's been that way for a long time.  

We'll, I actually tried to measure the length of my fish and estimated my tank to have 9 1/2 inches of fish.  

Back in December, I siphoned out and changed 20% of the water, then started using Easy Balance right after.  In case you don't know what it is, it is liquid you add that keeps water biologically balanced for 6 mo., stabilizes pH and alkalinity (KH)levels, reduces phosphate levels, and adds vitamins and trace elements.

We'll, you just change H20 lce every 6 mo., 50-75
% of it, using this.  I changed 8 gallons of water in the beginning of May.  

Back in March I got a fish with tail rot and treated it in a goldfish bowl.  In May I got a fish w/popeye.  At the end of June I think a fish got tailrot, it was hard to tell at 1st, the tail looked like it had a tiny curve. I  began treating it and stopped because the fish was always lying at the bottom of the goldfish bowl, so I gave up.  When it died, I saw some of it's tail gone. A few days ago, I got a fish that was whirling around and was bent, then it died.  Finally yesterday, I got a fish that had tailrot, and I began treating it and I think it whirled around before it died.  It was bent too.

I always put my sick fish in a goldfish bowl.  Also my water readings are always the same.  They read as ammonia 0, pH 8.4, alkalinity 300, hardness 150,
nitrite 0, and nitrate 20.  What could be wrong?

I spoke w/1 petstore guy and he told me to add
aquarium salt.  I looked at the directions and
asked if I follow the directions for conditioning
water and reducing stress or promoting recovery from fish disease.  He told me for promoting...

I added it in the aquarium while sick fish wasn't in there.  The directions for this say use w/medicine, I used no medicine though.  

Answer
Hi Dipan;

Snails contribute to the population too. They are pretty big waste producers. What I did was make a tank just for snails and move them to or from my other tanks as needed. Your tank is probably overcrowded because of these guys. It happens sometimes.

I am really not a fan of Easy Balance or anything else that discourages making water changes. Waste builds up and water quality decreases with time, no matter what you add to the tank. These products can't compensate for that in my opinion. The size of your fish is one indication that they don't have what they need. The other indication is the illnesses. Water changes will provide what may be missing. Remove 25% of the water every week, vacuuming the gravel at the same time. These products might be helpful between changes, but there is just no substitute for regular weekly water changes.

It helps to feed the fish a wide variety of foods. I like to give my livebearing fish chopped spinach, slightly cooked and cooled squash, broccoli, peas, and green beans. Fish even eat chunks of banana! Weird, but many of them like it. Remove anything that isn't eaten in 12 hours or so.

It is certainly up to you, but I hope you will consider making water changes more frequently. You will notice the health of your fish improve with the new foods too.

Followups welcome

At Your Service;
Chris Robbins

Come on over and join us on the freshwater fish forum at About.com to get even more information too;
http://freshaquarium.about.com/od/questionsanswers/a/naavigateforum.htm

My member name is ChrisR62. See You There!