Four ways in
Pick the subject, and it will send you back into the site
Chemistry, and the one number that decides the rest
pH as a ladder where every step is ten times the last, the four hardness units that all mean the same thing, and why iron turns to rust before a plant can reach it.
14 exercises
Physics, and why a wide tank breathes better than a deep one
The same water in two shapes does not get its oxygen back at the same speed. How far light reaches before a leaf stops paying for itself, and what the surface looks like from underneath.
12 exercises
Biology, and the things in the filter nobody can see
Who is really doing the work in there, what a leaf has to earn back to be worth keeping, and why the story the shop tells about two bacteria is older than the evidence.
13 exercises
Maths, and the arithmetic the rest of this site is quietly doing
Liters and millimeters, what is left in the tank after a water change, how quickly a number doubles when nothing leaves, and why pH is a logarithm.
14 exercises
How the exercises work
Every exercise is missing something on purpose
Each one gives you a situation, then the numbers you have been handed, then the question. What it does not give you is the formula. The relation you need, or the constant, or the threshold, is written out somewhere else on this site, and the exercise tells you which page to go and read.
So it is less a test than a reason to go and find something out, which is the part that tends to stay in your head afterwards.
Some of them check themselves. If your answer matches a figure already printed on the page you were sent to, you know you have it right and nobody has to tell you. Others finish in a decision rather than a number, and there the honest answer is the one you can argue for.
Start with a subject you enjoy and try one exercise at a time. You’ll use division in many of them; the later math exercises introduce powers and logarithms. For a gentler start, explore the aquarium stories.
The answers
They are deliberately not on this site
Somebody working through these should be able to reach the end without the answer sitting one scroll below the question, and a teacher or a parent setting them should not have to hide the page first. The worked solutions exist, with every intermediate value and the marking tolerances, and they are kept off the website on purpose.