zebra fish

Question
Have bought a new cold water fish tank about 12 weeks ago. We started out with 1 goldfish, 2 blackmores 2 golden sucking catfish and 2 zebras.  The catfish died about 1 week later. Replaced them with a bronze goldfish. that lasted about 2 weeks too. Replaced them with 2 more zebras. About 2 weeks later 1 of the original zebras developed a lumpy thing on its back behind its head. Then you could see red type lesions under its skin in its body behind its head.  This was going for about a week before it died.  We now have another zebra fish (could be the original) going the same way. Does not look very good. Can you give me some advise on what we could be doing wrong. Only feeding once a day
every four or five days then none for 1 from advise from petstore
after had problem with water.  We change 1/3rd water about every 2 to 3 weeks.  Are these fish dying because of ammonium if so what do we do to fix this.
Thankyou

Answer
Hi Kathy,
How big is the tank?  Since you have goldfish, they have a pretty high bio-load and require bigger tanks than other types of fish their size.  Also being a new tank without the adequate time to cycle, ammonia dn nitrite intoxication is a possible cause.  We wont know for sure unless you get a liquid test kit and test it.  Test strips are very inaccurate so be sure to get liquid testing.  Some common symptoms of ammonia intoxication is very fast breathing, and erratic swimming because it damages the nervous system.  It can also cause acute death which I believe is what you are experiencing.  On newly set up tanks that are being cycled with fish.  15%-20% water changes are encouraged every 3 days.  2-3 weeks is too long for a non-mature tank and will cause intoxication.
-Matt-