Problem with dying fish

Question
Hi Nathan,
I wondered if you could give me some advice on the fact that all 3 of my fish died in one day!
I set up my aquarium about 6 weeks ago with dechlorinated water etc and added a fantail and black moor.  After 4 weeks  everything looking ok, I went to the shop to have the water tested and they said the levels were absolutely fine.  I then decided to add another fish - a telescopic eyed fish which happened to be quite large compared to my other fish once I got it home.  I put him in and everything appeared ok for about a week and a half but then on wednesday evening I noticed white spots on him which appeared to have transferred onto my fantails tail.  All three fish appeared to be quite lethargic and staying close to the bottom of the tank by the following day.  I did a water and filter change (which I hadn't done up till that point) and on the saturday morning I took some water to be tested and they said that the ammonia was ok, but the nitrate was high and I should do water changes over the next 3 days.  Unfortunately by 5pm that evening the telescopic fish had died, by 12pm, the fantail had gone and by the next morning, my black moor had died.  I assume that I maybe should have done the changes when I noticed a problem on the wednesday? (was away on the Thursday, so didn't see them again until the Friday evening)  I'm also not sure if I was over feeding the fish and that caused the problem with the water within a week?  I have a 30 litre tank and maybe it isn't big enough for 3 fish?  Do you think the telescopic one was already ill? Maybe it needs more care?
Thanks, Jo Jackson
I am preparing myself to start all over again but obviously need some advice as to what I can do better/different the next time.  
Your advice would be appreciated as I'm gutted that everything seemed to be ok.

Answer
Hi Joanne,

Your Goldfish were actually sick. They had a disease, called Ich. (Ichthyophthirius multifilis) Ich, sometimes called Ick or Whitespot, is characterized by small white dots on the fish, as if it were sprinkled with salt. Ich is a parasite, and spreads rapidly in the aquarium.

For future reference, it can be treated with an anti-parasite medication. Medications for fish are commonly available at your local fish store. Ich is probably the most common disease in fish tanks. Most likely, the telescope eye was carrying it, and introduced the disease into your tank.

One more thing I would like to point out - Goldfish need a minimum of 76 liters (20 gallons). A 30 liter tank is not adequate for Goldfish. Why? Because of two reasons. First, goldfish grow to a large size. Common and various fancy Goldfish will reach an adult size of 12", and Comets will reach an adult size of 24"! They also produce lots of waste, which demands a large water volume to help control it.

True, you can keep Goldfish in a 30 liter tank, but they won't be very healthy, and may die a premature death. Keeping them in such a small tank can cause stress and diseases, which is what happened in your case.

For your tank, I recommend that you setup a small tropical tank. You could fit 6 Neon Tetras or a similar shoaling fish in there, like 6 Zebra Danios. You could even have live plants in the tank, since they do not eat plants like Goldfish do. The only thing that you would need for this would be a small 25 watt heater (around $15). The heater should be set to 78F.

When getting fish, inspect that they are free of white spots, red marks, torn fins, extremely bloated bellies, and that they are swimming properly.

During the first month of owning new fish, perform 10% water changes daily. After this month, you can switch back to a normal weekly routine of 20% per week. The beneficial bacteria that remove wastes will have already established at this time.

I hope this helps, and happy fishkeeping!