It is telling me I have a derivative of 14,000/s for a tank of 50,000 in size. Yeah, the graph also has no axes making judging the rate hard.

Connecting a container such as a tank to a rate combinator will indeed give ridiculous numbers. It will assume that the value received on each tick--that is, the contents of the container--is a separate event. So for a tank with 1,000 units of fluid, it will report 60,000/s, since it thinks 1,000 units of stuff is "happening" on each tick. You probably want to use a gauge combinator in this context.

So far as I know, there is no good way to measure the rate of fluid flow in a given system. Unlike with belts, there's no way to "pulse" when a unit of fluid enters a given pipe segment, for example.

To measure throughput you need 2 tanks side by side with pumps going in and out. Then fill one tank with the pump and empty the other with a pump while blocking the other 2 pumps. One tank measures production, the other consumption. Just before a tank gets empty/full you switch.

That still leaves you with an absolute value, not a pulse. So subtract the current value from the value last tick using a each+0=each combinator to delay the signal by one tick.

If you just want to know the delta between production and consumption a single tank with the pulse combinator will do.