A good one from my wife (user missgurnus): "You need to change out your filter media at least once a month!"
Explanation? Filter companies want more $$$$
^This^
Lets not just answer,lets discuss what the real deal is.
Some myths ,like many facts can have an exception.
The inch per gallon is pretty good if you are keeping neons,or cardinals.
10 neons in 10g,not best,not worst IMO.
10" Oscar in a 10g?
Lets discuss why the myth exist,or the misunderstanding behind them?
What about you can't change 100% water!!!!
The problem is that the rule doesn't give any hint or nuance to stocking. It works in such limited scenarios that it's almost worthless. It's good for 2 inch fish with a body shape like that of a neon. But then you have 3 to 4 inch fish like kuhli loaches that are long and thin and have a tiny bioload. A 12" spiny eel has much less bioload than a 12" oscar, which is much different than three 4" rainbowfish, four 3" platys, or six 2" neons. It also doesn't take into account diet- herbivorous fish tend to produce a lot more waste than carnivorous fish.
Ultimately the problem with this rule is that it enables people to stock their tank with minimal research- just add up the maximum sizes of the fish. Throwing that rule out the window forces people to research the diet, bioload and activity level of the fish in order to stock properly. It's a shortcut that leads to general fishkeeping ignorance (and in many cases actually badly limits stocking).
Most of my tanks debunk this rule pretty badly:
20 gallon "south american": five corydoras and fifteen glowlights. 45" of adult fish
20 gallon shellie: 15 adult 2" multies and a lot of babies. 30" of fish
29 gallon angelfish pair: 2 angelfish. Adult size 5"; 10" of fish
55 gallon community: (planned stocking) 1 angelfish, 5 platys, 5 kuhli loaches, 5 yoyo loaches, 5 angelicus loaches. A whopping 90" of adult fish!
I think that "you can't change 100% water" is true on a technical level- to actually do a 100% water change you'd have to remove the fish and substrate and completely dry out the tank, which IS generally not a recommended thing to do unless you're completely overhauling something.
They FEED the algae but do not CAUSE the algae. Algae spores are everywhere in the water ( fresh, salt, tap, well, ocean, land) and just need the proper environment for them to grow into the wonderful colored messes we all hate. ( Grrrr) So I think Sk3lly gets this one because of the word "cause". 
(My wife wasn't there to stop me from trying to lecture people like she usually does...)
