4 goldfish need no less then a 60 gallon tank.
Algea eating shark? is it a Siamese algae eater? They are well known to stop eating algae and start eating the slime coat of the sides of fish. They dont belong in a tropical tank with goldfish.
Any fish that eats algae will poo algae fertilizer. removing algae is only ever going to be a temporary fix equal to mowing your lawn. You have to find out why you have algae. many times the algae is the only way to know that things are not right in the tank.
brown algae - your tank may not be cycled.
other algae, -
you are likely over feeding, it is VERY easy to do , we all do it sometimes.
You may be over stocked or under filtered.
You may need to clean more, gravel vacuum more, remove all or most of the gravel so that detritus gets suck into the filter instead of settling into the substrate.
Tank lights should not be on more then 8 hours,
you may want to look for different lighting that doesnt support plant growth such as aquaglow bulbs.
Chemicles to kill plants hurt fish and should never be used!, Algae is a healthy even helpfull live plant!
NEVER remove fish from the tank! water conditioner works instantly! Use Gravel vacuum to change 25-40% of the water while sifting threw the substrate. Add the declorinater before OR WHILE ADDING NEW WATER. DO NOT REMOVE THE FISH, every time you touch, or the net touches a fish they lose slime coating "fish immune system" You can add a product like Prime Stress gourd, but It may not help completely. Every time you remove those fish you are adding them to a brand new environment that they have to get use to, it is VERY stressfull and you are lucky that they are still ok,
Get a digital thermometer to watch the temp when adding new water, or let the water sit out for 24 hours in jugs. BUT DONT REMOVE THE FISH!
Dont clean the filter once a month! You can rinse the filter cartridge in used tank water, or dechlorinated tank water. If you have 2 or more filter cartridges, only rinse one ever month, then let the clean one re build the nitrifing bactieria 4-6 weeks before cleaning the other. You can add filter media to the filter to hold extra healthy bacteria to make it even safer to clean the filter when needed. Filter media could be purchased like bio balls, or chips, or you can use any chemical free sponge, craft felt, filter floss.