It all depends on the "bio-load" of your tank - Bio-Load is the waste created by the fish and waste removed by the good bacteria in your filter.
If there is only a small amount of fish producing waste you should be safe with a fortnightly gravel vac
If there is a heavy bio-load (10-15 fish) you should be doing part water changes and vacs weekly.
All depends on what waste is produced and removed.
For a fish tank, you need enough gravel to make a thin layer on the bottom of the aquarium. For an outdoor walkway, you need much more.
Air bubbles will do nothing. What is needed is a filter.
I would think at least a 60 gallon aquarium
Gravel is not essential for goldfish, but it can provide a natural environment and anchor plants in the aquarium. However, make sure the gravel is large enough to prevent the goldfish from accidentally ingesting it, which can cause health problems. Regular gravel cleaning is also necessary to maintain water quality.
An aquarium, if you want plants, depending on the type you may need substrate, a filter, sand or gravel about 1 pound per gallon, a light between 5000K and 6500K (it does not have to say aquarium), water purifier, water, fish food, a power strip that has at least 4 outlets, an aquarium heater if you are going to have tropical fish. and about 30 minutes to put everything together and add water
10 gallon if its only 2 inches and a 20 gallon for 4 inches
I wouldn't advise it but I suppose it is plausible.
To determine how many yards of gravel fit into a 55-gallon drum, you first need to convert gallons to cubic yards. A 55-gallon drum holds about 0.2 cubic yards (since 1 cubic yard equals 201.97 gallons). Therefore, approximately 0.2 cubic yards of gravel can fit into a 55-gallon drum.
1 square km of gravel
you need more neighbor i guess, maybe you can try this site, is all about happy aquariums.http://www.yayeasy.com/
1 cement 2 sand 3 gravel
To calculate how many hours it will take to fill the 10-gallon aquarium, we need to convert the volume measurements to a consistent unit. Since 1 gallon is equal to 4 quarts, the aquarium holds 40 quarts of water. With 1 quart being added every 3 minutes, it would take 120 minutes (or 2 hours) to fill the aquarium.