A lid seems a pretty primitive solution plus it wouldn't help against feedback on the fish's position itself. Furthermore it would obstruct the vision of the ca

A lid seems a pretty primitive solution plus it wouldn't help against feedback on the fish's position itself. Furthermore it would obstruct the vision of the camera. If you're designing a craft if possible you want to avoid the movement of the craft changing the control inputs. If not, and I'm betting this thing already experienced that, it can get into a feedback loop, either creating very "shocky" movement, or unbounded acceleration. Instead, prevent having waves in the first place. Just like a person can move a glass without generating (large) waves in the liquid. What you want to do is to avoid using the position of the fish directly as input to the motor voltage. Instead, use it as an input to the control circuitry. Then calculate the force generated on the fish : voltage is linearly related to speed. And the differential of voltage gives you the force on the boat/fish. So let the fish move and this sets a "target", then every timeframe you bring the voltage, say, 0.01V closer to what the fish "wants" to set it to, limiting the force exerted on the bowl. Btw: there will be a "resonant" value that will be relatively low in value, but will build massive waves. Avoid at all costs. In a project like this I might just assume I'm very unlikely to configure just the resonant value, but in a real project I'd make DAMN sure I knew what the resonant value(s) is/are and put in a warning. You probably think that this will restrict the movement a lot but it won't. Find the right values and you will hardly see any waves at all.