I have to disagree with Byron and say frozen foods are nutritional. Freezing food for long periods of time can cause them to lose some nutritional value, however they don't lose much. If they did we wouldn't have so many frozen foods available to people, meat, veges, icecream (not necessarily nutritional but yummy).
Freeze dried foods are also nutritious. They use a low temperature to dry the food out and that preserves most of the nutrients.
High temperatures (boiling and cooking) will destroy vitamins in food, and the temperature can either increase or decrease the nutritional value of some foods by making them easier to digest. But for fish keeping purposes, frozen (but defrosted) and freeze dried foods and live foods are fine and contain lots of nutrients.
----------------------
Dry flake food can go off very quickly once opened. They absorb moisture readily and damp flake food can go mouldy and lose its nutritional value very quickly. You might notice when you first open a container of flake food that the flakes are hard dry and crispy and crumble up easily. After a few weeks the flakes are softer and don't crumble as easily. This is due to moisture in the air being absorbed by the flakes. Because flakes absorb moisture rapidly when exposed to the air, you should try to use all the flake food up within a few weeks. The easiest way to do this is to use small containers of food but that costs more money. Opening a container of flake food and putting most of the flake into a small plastic zip lock bag, squeezing the air out and sealing it up, will help keep the flake dry and fresh.
You can put a gel pack (moisture absorbing pack) into the flake to help keep it fresh.
Try to keep dry food away from the aquarium and out of humid environments.
Try to get a dry food that does not have cereals (wheat) in them. Fish can't digest grains and some companies add fillers (flour from grains) to bind the other ingredients or to save money. A basic flake food should contain fish, prawn and maybe other aquatic organisms.
For small fishes like tetras you should crumble the flake up so it is easier for them to eat.
----------------------
For small fish like tetras you can use newly hatched brineshrimp and culture your own food at home. Buy some dry brineshrimp eggs online and keep them sealed up in an airtight container in the freezer. Take out a small amount and hatch them while the rest of the eggs remain frozen. There is more information about culturing live foods at the following link.
http://www.fishforums.net/threads/back-to-basics-when-breeding-fish.448304/
Daphnia can be found in ponds that appear in winter and spring and can be used to start a culture at home. Or you can buy dry eggs/ cysts from Aquaculture supply stores and use them to start cultures. Rotifers can also be cultured in the same way as Daphnia and their cysts are available from Aquaculture supply stores too.
Microworms will be another good food for your tetras. There is info about culturing them at the link above.
Grindal worms can be cultured in peat or potting mix and fed on dry powdered baby cereal.
Mosquitoe larvae frequently inhabit water containers during warmer weather and these can be scooped out with a fine mesh net, rinsed and fed to the fish, or frozen for use in winter when mozzies are not around.
Aphids can be collected from roses and fed off or frozen for later use.
Midges (small flying insects) can be found in swarms around water during spring and summer. You can use a fine mesh net to catch them and put them in a plastic bag before freezing them. Then feed them to the fish.
Chirominid Midge Larvae (aka bloodworms) can be cultured in the backyard. Put a container of water outside under a tree and add a handful of non toxic leaves. Wait a month and eventually you will see small red worms about 1/2 inch long wiggling about on the bottom. They build a house out of small particles of plant or sediment and live there until they metamorphis and turn into midges.
Bloodworms have a very hard head that cannot be digested by most fish, and live bloodworms have been known to chew through the stomach wall of some fish, usually those that don't chew their food before swallowing. Because of this, you should either freeze bloodworms or remove their heads before feeding them to fish.
Some brands of frozen bloodworms can cause problems to fish. Hikari is pretty safe and I have not heard any issues about that brand. Check all frozen food for frost damage (white frost on the food) and for defrosting (food usually looks different to normal). Avoid frozen food with damaged packaging or frost damage or food that has been defrosted and refrozen.
----------------------
If you can't get frozen fish food, you can make your own. You can put newly hatched brineshrimp into ice cube containers and freeze them, then store them in an airtight container in the freezer. You can do the same with Daphnia, rotifers and mosquitoe larvae.
You can buy fresh or frozen prawn/ shrimp (raw or cooked) and keep that in the freezer. Take out one prawn and remove the head, shell and gut (thin black tube in body) and throw those bits away. Then use a pr of scissors to cut the prawn tail into small bits and offer 1 or 2 bits at a time. Feed a few bits at a time until the fish are full.
You can also use raw fish, squid, octopus, mussel meat as well as prawn. If you have a meat mincer, you can put fish, prawn, squid and other types of marine organisms into it and make a mixture. Then put that into plastic bags and spread it out into thin sheets (3-6mm thick, 1/4-1/8inch) and freeze them. When you want some food, you break a bit off and defrost it and offer a little bit at a time.
I use to run the mixture through a mincer 3 times for my fish but it depends on the mincer and the fish being fed. For tetras and other small fishes, mince it several times.
----------------------
Baby guppies make a good live food if you are that way inclined. Feed the babies really well with a good quality flake food and then feed the guppies to other fish.
Glass/ Ghost shrimp will breed in aquariums and their young make excellent fish food too.
You can feed live or frozen fish foods as often as you like and most fish do better if they get frozen foods several times a week. I use to feed my fish dry food in the morning before work. In the evening I would feed dry food followed by frozen food and then live food. If you are trying to breed fish, then feed frozen or live food every day for several weeks before you breed the fish.
