So I've already broken 1 of the 4 tubes that came with my kit and it sucks having just 3. The replacement API tubes are expensive. I found these on Amazon and at an excellent price. They are similar in size and one reviewer said they used them as a replacement to the API tubes. Has anyone else purchased plastic tubes like this? Any reason these couldn't work? They also don't have flat bottoms, but I don't know if being upright is even important and if it is I can always place them in a cup. Obviously there is no 5ml line but I use a syringe to fill mine anyways. It's much easier!