Im fully in agreement with severum mama. I've seen far too many people reference the 'rule' and they're only setting themselves up for failure. The way I see it, a rule is a rule. If there's a rule saying you can't run down the sidewalk after 10pm, one would assume that you can walk fast, jog, skip, ride a bike, etc. So, if there's a 'rule' that says you can have 1" of fish for every gallon of water you have in your tank, a beginner who has never researched fish may think it's fine to get a 10" fish for a 10g tank. That's what the rule says, right? I would say that 1" per 5g is more appropriate, but there are still exceptions. Fact is, there will never be an end all stocking rule.
Tools like aqadvisor will never be able to give accurate advice. Sure, it will steer you away from an oscar in a 10g tank, but it still won't give what experienced hobbyists can. You also have to take in to consideration that different fish have different termperments. What works for me, may not work for you. What works for everyone else may not work for me. There's just no replacement for experience... period.