The Grammar Aquarium is part of Auto-English, an excellent page that contains free online and print-out exercises. Although there are exercises for all levels, it is 2nd cycle and Batxillerat students who will take good advantage of it. Besides the Aquarium, watch out also the different sections under Exercises: idioms (The idiom jungle), mistakes (The boo boo desert), phrasal verbs (The phrasal verb picnic), vocabulary (The vocab pool) and writing.
All the exercises are classified according to grammar topic. We’ll start by doing some questions. Scroll down the page until you find the section, or choose Questions from the menu on the left hand side.
When the ten sentences have been completed, the program shows the score.
Click on this bar at the top of the page to go back to the exercise menu.
In case of doubt, the Hint button at the bottom of the page
It will help you by providing the first letter of the correct word. When you finish, check to get your score.
Click on the arrow to go to the next sentence.
Click on Check before you move on to the next sentence.
Click on Show answer or Hint if you need help.
The arrow on the right takes you to the next question.
To enter the words, click on the number. This will show you a clue and a blank to write the verb. When you click on Enter, the word will be added to the grid. If you need help, the Hint button will give you the first letter. Check to get your score.
Other good pages with exercises at a similar level are
http://www.englishpage.com, Free online English lessons & EFL resources.
http://www.usingenglish.com, Resources for Students Learning English: nearly 400 grammar and vocabulary quizzes, articles on English usage and grammar reference.