The food didn't have anything to do with it. Out of date fish food simply has a lower nutrient level (making feeding it pointless) but it wouldn't kill them.
Firstly, your tank was likely a 10 gallon tank. The fish you got must have been some kind of livebearer - probably platies - and the ones with the feelers would have been gouramies.
First of al you probably had too many fish for the tank - though I can't say without knowing the exact species of fish. In future, research what fish you are going to buy and make sure you never impulse buy. Most fish start out life at about the same size - many will then grow to over a foot long or are too fragile or have specific requirements.
Which fish survived? Can you describe it or post a picture?
You say you added 'bacteria'. What was the brand name? Most of these products are useless.
Regardless, if you added the bacteria and then left the tank to run for a few days, the bacteria would have died anyway by the time you added your fish.
You also added too many fish, too quickly.
I suspect you have never heard of cycling.
Basically, fish produce waste. Waste contains ammonia. Ammonia is deadly. Good bacteria grow in gravel and filter media. Good bacteria eat ammonia and turn it to nitrIte. NitrIte is also deadly. More good bacteria grow in gravel and filter media. They eat nitrIte. They produce nitrAte. NitrAte is harmless if kept at low levels. You take out nitrAte by doing weekly 25% water changes with de-chlorinated water.
This whole process takes about 6 weeks with fish. You are supposed to monitor ammonia and nitrIte. When they are ZERO, the water's safe for fish. Anything higher is toxic. If you test the water during a cycle, you'll see that after about a week the ammonia reaches its highest concentration (it 'spikes') and this is the most dangerous time for fish. Afterwards, good bacteria convert this ammonia to nitrIte and eventualy the nitrIte also spikes while the ammonia is decreasing. Finaly, ammonia and nitrIte both reach 0 and the water is safe. The nitrAte is high and you need to do water changes to keep it under 40ppm - prefferably under 20ppm (parts per million). Your test kit will show you when the tank is safe for new fish.
In your size tank, the one fish that has survived is all you need to cycle it. If it is a gourami, however, I would take it back to the LFS and do a fishless cycle (I'll explain later) as they are more fragile than platies. Once ammonia and nitrIte are at zero, you can think about adding more fish - but only once the tank is fully cycled and you have researched the species you are after and only if they are compatible with the fish you already have.
You can speed up the cycling process by going to your LFS and asking for some gravel and/or filter media from one of their HEALTHY established tanks. This will introduce some of the beneficial bacteria to your tank and give you a good start.
If you decide to start over, I would do a fishless cycle. What this involves is adding ammonia to the tank in an artificial way (ie not from fish waste
). You can either add pure ammonia like that used for cleaning or add fish food daily as if you have fish. The fish food will rot and ammonia will be produced. From this point on, the cycle is identical to that with fish. You still have ammonia bein converted to nitrIte and then nitrAte and the usual spiking. However, fishless cycles are MUCH quicker as you don't need to worry about the fish' well-being (as there are no fish) so you don't need to do water changes (which decreases the amount of ammonia/nitrIte and so reduces the amount of food for the bacteria and slows the process). It is also a lot less stressful for both you and the fish and you aren't risking anyone's life!
There are links in my signature concerning all this stuff so please do read them.
Oh and before I forget - use de-chlorinated water for water changes (I suggest you do a water change every other day now that your tank is cycling with fish and monitor the parameters in case there's a spike - then do another water change if the levels are too high). The chlorine added to tap water kills your good bacteria so using tap water directly with removing the chlorine kills of the good bacteria in your gravel and filter. Similarly, don't wash your filter media in tap water. If the filter gets clogged up, use some old tank water in a bucket to carefuly rinse all the muck out. Avoid changing your filter media - ever - unless it starts to disintegrade, there's no reason to do so. If it becomes incredibly bad and rinsing won't do the trick, replace one bit of the media at a time - over a period of 2-3 weeks.
Good luck
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