Which fish you have?  

I had many different kinds of fishes,for almost 7 years.I definitely miss them now.

I don't know why your fishes died.I think, you know that excessive food would cause the death of fishes.

Basically,the fishes can be classified into two categories.

#Soft variety and

#Hard variety

Soft varieties includes all fishes which do not bite or cause harm to the other fishes in the aquarium.For example,gold fish belongs to soft variety.

Hard varieties are aggressive in nature and bites other fishes.Fighter is a hard variety type of fish.

You can have either a set of soft variety fishes,or a hard variety type of fishes.If you leave a mixture of both soft variety and hard variety,then the soft varieties would be bitten and even killed by the hard variety type of fishes.

Well,I recommend you of having some soft variety type of fishes,and don't over feed them.

Cheers ! Good day to you and the fishes you are going to have.

There are cold water fish and then there are those that need warmth to be comfortable and stress free, they should not be in the same tank as too warm or too cold a temp will stress the fish and lower their resistance to diseases. And then you need to have tanks that are large enough for the fish, just because the fish are not bumping into each other does not mean that there is enough room in the tank. some small fish such as the neon tetras need lots of swimming room so that should be best kept in a tank of at least 20 gallons or more in "schools" of 6 or more to make them feel comfortable. Also in small tanks , they get polluted quicker, that means that ammonia, nitrite and nitrate will built up faster. YOU need to have a tank that is "cycled" fully to have a full fish load in it. Also remember that the only oxygen that is getting into the water is through the surface of the tank, think of yourself having to breath through a straw, the larger the straw the easier it is to breath through. The same applies to a tank, the larger the surface area the more oxygen can be taken in. If you need more information go to aquariumpros,com and they will give you lots more information to help keep your fish alive.

Currently we have about 50 big Japanese Kois, some small tin foil barbs, some black mollies, 13 bala sharks about 2 fighting fish and some other fish ...oh right we have some cichlids too but they are aggressive ones so we had to separate a few of them.
Not to long ago I had over 500 fishes to take care of. The included blue dolphins, speckled mollies, flamingo guppies, albino and black tiger oscars, a flowerhorn and lots and lots of Siamese fighting fish. Bet I forgot a few of them too...
When I got a job I didn't have time for them all so i sold most of them and kept the ones that are easier to maintain. I got to admit, very rarely a fish would die on me but when one did, it felt so sad! It's hard to watch a fish grow up and then have to see it go. But that is their life cycle too, nothing we can do sometimes.

I used to keep Gold fish till I found out just how big they should grow, and how long they should live for. (Over 8ins and over 20 years)I also learnt from a fish-expert just what happens to them when kept in the confines of a small tank or bowl. If they weren't as tolerant of mistreatment or as cheap as they are, I'm sure less people would keep them in anything under a 4ft tank or a pond.
Now I only keep Tropical fish and shrimp of various compatible species and I've a tank of African Dwarf Frogs, which are fully aquatic.
If you kept gold fish and a fighting fish (male Betta) together then I'm not surprised they didn't survive! They aren't compatible. One is cold water one tropical. The fighting fish will attack anything that resembles another male, so if your goldfish was a long finned variety it would be bullied to death by the aggressive Beta. The Betta couldn't survive the treatment that a gold fish would. Too large a water change and cleaning a tank too often can cause a great deal of stress to fish and that's when diseases strike.
Always try to learn as much as you can about how to best keep a fish or any other pet before you buy one and then you'll have happier and healthier ones that will live to old age!
Even though I've been keeping fish for over 40 years and remember having a pet goldfish over 50 years ago, I'm still learning! There are so many new species available I would like to keep, and I'd like to keep Marine fish, but at present don't think I know enough or have the time to provide the care they need.
I go onto several of the fish-keeping sites and ask lots of questions though and when I retire, who knows?