AqAdvisor

I personally think its a bit conservative in stocking levels. It also does not take into account many variables

AqAd is a GREAT site if used properly. It will give you heads up on possible issues, but the leads need to be followed up on and looked into further. It doesn't take things like plants into account, along with a few other things. Use it as a place to start, but not as a definitive answer to all life's questions.

I've messed with it a bit. As a general rule, you can go up to about what the site says is 110%, at least judging by what it's told me. It's not going to be absolutely right, but it's great for seeing about how stocked the tank is and if you have any compatibility concerns.