I upped my usual water tests to 8 tests and have several reservoirs that required about two dozen tubes. The 5ml scale is accurate for the tests provided by API but their caps tend to leak so I went ahead and bought the regular rubber stoppers and they work great. Included is a link to stubs that are a good fit. Pack of 25 - CleverDelights Solid Rubber Stoppers - Size 000 - 13mm x 8mm - Length 21mm - Black Lab Stopper #000 https://www.amazon.com/dp/B01LZWPWFN/ref=cm_sw_r_cp_apa_i_UyudCbSB2ZX9Q
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