It's just a fish tank. Pests keep drifting down toward the shrimp, and each one carries a word — type the whole word to scoop it out before it reaches them. Your first keystroke picks which one you're cleaning.
Calm = slower, fewer at once. Open water = faster. Backspace lets go of the one you picked.
Every wave you clear pays 2 Witcoin in Calm current, 5 in Open water. Finish with 90% accuracy for a bonus (+3, or +6 in Open water), or 95% for a bigger one (+5, or +10). One run pays up to 75; Tank Defense pays up to 350 a day.