An ultraviolet steriliser (UV light) won't make any difference to this. Save your money and send it back or cancel the order.
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Garlic in fish food doesn't do much and is a bit of a gimmick. Fish food manufacturers put things in fish food that people recognise (eg: wheat flour, garlic, rosemary, vitamins). Most of these things are useful to people but not fish. You can try it and it might help but fish never evolved to eat terrestrial plants, let alone terrestrial plant bulbs like garlic.
Goldfish need lots of plant matter in their diet and you can grow duckweed (small floating plant) and other aquatic plants like Ambulia outside in plastic containers. Each week you bring some of the plants in and put them in the aquarium for the fish to eat.
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If you add a 1/2 to 1 inch layer of fine gravel to the aquarium and get a basic model gravel cleaner, you can gravel clean the substrate (gravel) each time you do a water change. Don't buy an electric gravel cleaner for an aquarium, they are rubbish and a waste of money. Just get a basic model gravel cleaner like the one in the following link and use that to drain water and clean the substrate at the same time. You can also make a gravel cleaner out of a plastic drink bottle and garden hose.
Aquarium cleaning should take 30 minutes every two weeks. Learn how the professionals do it in 18 easy steps.
www.about-goldfish.com
Home made gravel cleaner
Get a 1, 1.5 or 2 litre plastic drink bottle (like a Coca cola bottle).
Cut the bottom off the bottle and throw the bottom bit in the recycling bin.
Remove the cap and plastic ring from the top of the bottle and put them in the recycling bin.
That's the gravel cleaner part done.
Put a garden hose into the top of the bottle and run the hose out the door onto the lawn. If you don't have a lawn, you can use a shorter piece of hose and drain the water into a bucket.
You can buy clear hose from a hardware store and either get a hose that fits snuggly into the top of the bottle, or over the outside of the top of the bottle. The bigger diameter hose on the outside will drain the water faster and is better for big tanks. A hose that fits into the top of the bottle is better for smaller tanks.