| The test kits measure free dissolved copper (both Cu+ and Cu++). If you can measure it with the test kit, then you probably need to worry about it. They have demonstrated toxic effects at 30 ppb so Henry's suggestion of keeping copper at single-digit parts per billion is probably a good idea.
Bits and pieces of the KDF media and precipitates containing copper will not stay in the water column for long so they will not show up in the test kit results. Copper ions also have an affinity for organic debris and will quickly stick to any piece of gunk that it comes into contact with. Therefore, much of the copper will be found within the solid waste and, in a really clean system, it will be discharged with the solid waste. But, if there are mulm accumulations there could be pockets where conditions are acidic enough to dissolve the precipitate and release free copper. I believe ammonia will also disassociate some copper compounds. If the fish or fish keeper stirs up these acidic sediment deposits there could be a sudden release of free dissolved copper to the overlying water.
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