I wouldn't take too much stock in aqadvisor filter capacities, they are incredibly cautious. It's an alright indicator but if you go over 100% the tank isn't going to suddenly crash and it completely fails to take into account all of the surface area in the tank its self or anything like plants.
E.g. 200 Litre tank bare bottom with a fluval 306 and a 200 litre tank, 1-2" of substrate like aqua soil, fully planted and a fluval 306 filter = the same result on aqadvisor..... They are clearly not the same.
I'm not saying ignore it but I am just saying you need to apply some common sense to the equation. Assuming the Fluval 306 is full of media in it then it should be fine if you add a few more small tetra type fish similar to what you already have.
Just think when you see a old school fish store running small holding tanks crammed with fish on 1 or 2 small sponge filters. They aren't using any voodoo to achieve that, its just most people overestimate how much surface area is actually needed and under estimate how much is already in the tank.
I should also add that there is zero actual scientific research published about how much surface area is needed per type of fish or per gram of dried food. Pretty much all we have to go on is marketing and what hobbyists have been conditioned to think over time.
There are just so many variables to take into account (how much food, how often, type of food, tank decor, how much substrate, type of substrate, how many plants, the type of plants, how much light those plants get, how much ferts those plants get, use of carbon), I could go on but you get the point.
Aqadviser is just one persons opinion and isn't actually backed up by any research. Most people over spec there filter and are extremely cautious in this regard but its almost all down to how hobbyists have been conditioned and marketing.
Edit:
According to aqadvisor I only have 66% of the required filter capacity, not a brag.... just saying.