Your tank is overstock believe it or not. It may not seem like it ... but goldfish have huge bioloads so they really dirty up their water... and comet goldfish really belong in a pond rather than a tank. The general rule is 20gal for the first, 10gal for each additional. I know ... a big contrast to the dozens of goldfish you see in a tank at LFS
's. If you can upgrade to a 29 gal
that would be easier for you.
As for Alkalinity ... If your fish have been fine and healthy thus far ... don't mess with it. Trying to achieve that "ideal" level we'll read about can lead to pH fluctuations which will make algae the least of your problems.
I agree with your don't add extra chemicals to raise alkalinity / pH ... there are natural alternatives like crushed corals / limestone in a media bag.
For now, continue with the blackout for a few days and PWC
's. Algae grow from excess nutrients and light. Is there usually any uneaten food that remains? That along with their bioloads could be a source of the nutrients .. PWC
's will help. As for the lights ... consider going on a 4hr on 4hr off cycle, many members have had success with this.
75g Planted Barbs Denison, BlackRuby, Tiger, Albino Tiger, Green Tiger, Cherry, Gold, Checkered, Odessa.
NonBarbs Cardinal, Neon, RummyNose, Bloodfin Tetras, Z. Danios. 1RTS. Amano, Bamboo, Ghost & RCS. 20g Long Shrimp Tank RCS, Tiger, Amano.