Aggressive goldish?

[Kaikoura]

When my younger cousin gave me her 5 year old Red-Eared Slider, I looked up on some sites as to what they eat and so on. Going from that, I got reptile pellets and such for him, as well as some aquatic plants. I also got 6 feeder goldfish to feed to him as treats now and again.
When I first acquired the turtle, he was in a 5 gallon tank. Not long after, though, I moved him over into a 20 gallon Rubbermaid storage container. (I washed it of factory chemicals, and it's serving as a substitute until I can actually afford a much larger tank.)
What I did with the five gallon was set it up for the feeder goldfish. Keeping with the advice that I got from my mom to have no more than one inch of fish (nose to tail) per one gallon of water, I only ended up putting four fish in the five gallon tank. The other two went in with the turtle. Consequently, the turtle only ended up eating three of the six fish that I had. Currently I have one fish in with the turtle, and one fish in the five gallon tank. Here's where my question comes up..
Before all of this started, I had already fed the turtle, by hand, two of the fish. This left three in the tank and one with the turtle. Of the three in the small tank, two of them were the same size, and one was a lot smaller. After a little while, I noticed that one of the two larger fish was slowly becoming injured, but the other two remained healthy. This one fish began losing scales, it's tail became frayed, and it was eventually left with only one fin. While the fish was still alive, I did it justice and fed it to the Red-Eared Slider. It was only a day after that the smallest fish in the five-gallon tank began showing the same thing. This was before Christmas. I left just this Monday (Christmas Eve), and came back on Thursday. The white fish was completely dead, and it was obvious that it had been picked at even afterwards. Now the only fish that remains is that one larger goldfish. And it's completely healthy. I don't understand why the fish has acted so aggressively towards the other fish. I thought that goldfish were community fish and didn't tend to act aggressively.
I am interested in buying a fish that I am going to keep, and not feed to my turtle, but I am also going to buy a couple more goldfish as well. I don't want any of them to act aggressively towards the fish that I want to keep, or kill the other goldfish. What should I do?

[Ichthius]

Your goldfish are not aggressive.

With respect to your mom's advice, it's not correct. It's a common mistake to follow the "one inch of fish per gallon rule." But it's simply not a reliable way to stock a tank.

The problem that your fish died of was bacterial infections or ammonia caused by the cramped, overstocked tank did them in. Fin rot causes the fins to fray and deteriorate until the fish succums. You see, goldfish grow to be very, very large, especially the feeder variety, when kept properly; a foot in length or more. They are also very messy fish. A 5 gallon tank is too small for even 1 goldfish of any variety and only leads to a soon dead fish.

If you're not attached to the fish and still want to feed them to the turtle, you might want to give it another go. Otherwise, you're in for a big tank or opt for a pond.

[Kaikoura]

But the fish is showing absolutely no signs of the same problems that the other fish have shown. It's utterly healthy.

[emc7]

A five gallon tank is very close quarters, of course they pick at each other and there is nowhere to run in a tank that little. Goldfish are also omnivorous and they will eat dead (and dying) fish. You survivor survived the cycle and was resistant to infection (a more likely cause of all fins fraying that being chewed on) perhaps because it had survived those diseases in the past. Don't put any more fish in the 5. If you keep up water changes, you will be amazed at the growth of your fish in a few months. In the mean time, start asking everyone you meet whether they have a pond where your pet could be happy.

[Kaikoura]

Well, despite all of that. I only have four tanks. The biggest one is the five gallon tank. I have been looking for larger tanks, but I don't have a lot of money. Because of that, though, I have been willing to buy used and older tanks, even if I have to reseal them. The bad thing is.. I can't find any.

[SueM]

Kaikoura, keep an eye on Craig's list for your area, I find losts of tanks there...
http://craigslist.org/

[redpaulhus]

Originally Posted by Kaikoura

But the fish is showing absolutely no signs of the same problems that the other fish have shown. It's utterly healthy.

When you had 3 fish dumping waste into the tank, things were so dirty they were getting infections and suffering from fin rot (if ammonia is high enough it will literally burn their fins).
Now that there is only one fish, things are still cramped, but there isn't sooo much ammonnia that its burning this fishes fins.
That doesn't mean the fish is healthy per se, it just means that one of the visible signs of overcrowding is no longer present.
As this fish grows, you'll be back in the same boat.

As people mentioned, upsizing to a bigger tank is the best bet.
In the meantime, lots of frequent partial water changes will help (ie a qtr of the tank every few days, make sure the new water is the same temp and has been dechlorinated).

Also - if you can't find a used tank cheap, an empty 10g tank is usually about $10-$12. You should be able to use the filter etc from the 5g until you can buy a bigger filter. Or, you could pick up another $5 20g rubbermade tub and use it as an indoor pond for the fish....

[Kaikoura]

I actually considered doing such with the Rubbermaid containers until I could find glass tanks to fit my fish, redpaulhus. I know it won't be all that great for a display, but it would suit my needs until I have the means of upgrading it.
I have, actually, two more thanks than just the ones I am using. They're exactly the same, 4.5 gallons, and have a hexagonal shape, but with a standard flat back. They're both plastic
I keep thinking to myself, "Man.. If only I could just combine those two together..."

An indoor pond sounds like a great idea, though. We're going back up to the store later on today. I'll think on it and then decide when we go up there.