Why? You get what you pay for.... There are bargains out there... but generally.. if you pay next to nothing, its the quality that suffers...
For the sake of £2-£3 i would stick to buying stuff that has lables, ingredients listed and is known for its quality.
Fish will eat anything if they are hungry enough, doesnt mean it will keep them happy or healthy.
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That aside, for frozen foods it can be made easier and go further!
I bought TMC slabs of mysis, bloodworm and artemia (call arounds shops, most MAs sell them) and they are about £4 for a good slab of it...
I then blended the three slabs with some raw garlic, a little water to make it spread out, i added some brocolli buds or chopped seaweed sometimes... and a good bit of cooked and peeled frozen prawns... then i whizzed it all up in the blender, adding a little water until it was like a paste...
Then put it into the thick sandwhich bags and spread it out into a sheet and stuck it in the freezer to freeze into new bulked out slabs of mixed frozen food that i could make up and change quantities each time, was really easy and took all of about 5 minutes to do. Prawns make excellant bulk for the mix... cheap from supermarkets.