I'll have to echo what Inrun said Im afraid. There is a lot of debate on exactly how much space needed per fish per species, but I think you would be hard put to find anyone who isnt trying to sell you something that would disagree when I tell you that your tank is far far too small, mate.
And not just for the fish to move, but also for good water chemistry. The more water a tank contains, the more stable its water will be. Less water means that the levels of dangerous chemicals in the water such as ammonia and nitrates will fluctuate very quickly. As will the waters pH. Fast changes of water chemistry are actually far more dangerous for fish than slow but large changes.
I would advise at least a 10 gallon aquarium, and say to change 1/3 of the tanks water every 2 weeks.
You might also consider buying a different filter. Undergravel filters are, to be quite frank, terrible. They work ( quite badly in most cases ) by pulling dirt through the tanks gravel and holding it under the bed. This isnt a very good idea unless you have a good system to them remove it. Most people dont, and its not easy anyway. A better options would be a small cansiter filter, or even a sponge filter running on a small airpump. Both are reasonably inexpensive options, and will save you time, money, and possibly anguish in the long run.
As for feeding, the number 1 killer of fish by new keepers ( after stress ), is over-feeding. Once a day is fine, and so is twice a day. Even 3 times a day. The point is that in all 3 cases, the total amount of food should be the same. If you want to give them a decent feed in the morning and leave it at that then fine. This is what I do with all of my fish. You can split the ration into halves and feed them morning and evening if you wish, and some people believe this is a more natural way of doing so.
As for the amount, try not to go on the tubs directions. Ive seen some that advise the buyer to feed 5 times a day. These people just want you to use the product as quickly as possible and buy more. Sadly, as you will probably now notice with your tank and filter, this sort of attitude isnt uncommon when it comes to fishkeeping equipment, food and even fish.
The best way is to feed slowly a few flakes at a time unstill they dont want to eat any more. Some fish are just greedy ( Goldfish are famously so ), so use your judgment.
Thinks thats about it. Ive been keeping fish of all shapes, species and sizes for quite a few years now, and Im obviously not the only one on PPT. Dont worry, mate. We'll help you keep your friends swimming