Sick fish - random deaths

Question
I am a beginner and have been taking advice from my local tropical aquarium fish store.  They have been pretty good for the most part (much better than petsmart).  However I have had problems lately and they havent been as helpful.  I have a 20 gallon tank with 1 black and white ghost, 2 tuxedo swordfish and a sucker or algae eater fish.  The tank was really good for a while and no one died.  Then we found a dead fish and over the past weeks more and more keep dying.  A couple of the fish starting acting different, hiding in the corner all the time or not eating and we expected they might die.  Sure enough the next day or two and they were gone.  But most of the others that have died have just randomly died.  They seem fine one day and gone the next.  After asking the fish store several times and them not sure, finally one day one of the guys asked if the fins were droopy or there was any white on the fish.  None of their fins look droopy, but the 2 swords have some white under neath their head area.  The fist store gave me a quick cure bottle of blue stuff that says fast releif for ick and other parasites and then a bottle of kanamycin pills.  They have me do a 5 day treatment of the blue drops and 2 pills every other day for 3 times. (eg fri, sun, tue.)  This is now the second treatment and the swords are still white, but the others look fine.  However, yesterday the panda catfish died and he had seemed fine.  He's already mostly white so i dont know if he was sick.  Thats problem one.

I have a 2nd 20 gallon tank with 1 african dwarf frog, 6 red nose rammey, 1 guppie and a ghost.  The only time anything died in this tank was when we had a green tiger barb that turned mean and bothered the little guys.  We took him out and everything was fine.  We had 1 frog die, but we always wondered if something was wrong with him because he never grew.  We got two frog about the same size, one is still growing and much bigger but the other never grew and stayed real small.  He got bloated and we treated him.  I thought he was better and then about 4 days later he died.  Now recently we had a neon in there that died.  He seemed fine and didnt appear to have any white on him and he was droopy.  I'm guessing he died since he cant get out, but i never found him.  I looked all over and he's just vanished.  He's been gone a few days now.  
Problem 3 is that we bought a 55 gal and it is about 3 weeks old now.  We bought it on 9-15. The amonia is good, the nitrite is a little high but getting better and the ph is very high.  the fish store has us feed them every other day.  Today we found our first dead fish in this tank.  He was a blood red gourami.  Unfortunately, he came from the first 20 gal tank that we found was sick.  At the time we used him as a starter, the fish store hadnt helped us with the medicine stuff so we didnt realize they were sick.  now i dont know if the gourami died from being in that sick tank or that the water isnt good yet.  None of the other fish in there seem different.  We have a couple blue gouramis 1 red one left, 1 striped red one, a dragon fish and dinosaur eel and an albino shark (i think thats his name).  how do i keep my fishies from dying?  its so sad.  any help appreciated.  i look having fish, but i hate that they keep dying on me.

Answer
Hi Michelle,
Thank you for your letter. Sorry for the troubles in your tank. What a hobby this is. I believe you have a case of over feeding. Read and print this page:
http://steamboats.com/aquarium.html
I suggest that you stop all feeding for 24 - 48 hours, buy a bottle of Cycle (friendly bacteria), and follow the instructions at the bottom of the page.
You can help your fish if you put them on a strict diet. Cycle is necessary to start to break down the pollution in the water.
Write back if you want to discuss further.
Nori