With potassium, as with nitrates and phosphates, you'd have to try really, really hard to add enough to do any damage to your plants or fish. Macros aren't an issue. It's perfectly safe to dose standard EI doses and beyond. If you think you may have an issue with potassium deficiency, you can generally increase all macros with no danger. If it doesn't do the trick, you have another issue, and you can drop macro dosing back down to standard levels after a couple weeks. That said, if you want to increase potassium specifically, you can add K2SO4 to the mix of KNO3 and KH2PO4, without reducing the application of the other two. Again, however, drop it back down to normal if there's no positive results within a couple weeks.
The only current argument regarding overdosing EI involves micros, which is a whole different story.
Just make sure that if you're doing EI dosing, you're including the 50% water changes weekly. If you're not doing that, you're not using EI dosing.
But, I've never used a potassium test kit, and I'm not sure API (which is the most common brand of test kits we use) makes one.