My friend if your solution is getting darker after storage especially that short time then theres a problem. You might find that its the metal saucepan your heating the water in (maybe the coating is removed a little). It may be that you have it too close to a power supply or electrical outlet nearby etc. You need to keep Colloidal Ionic Silver away from any magnetic/electrical fields so note where your storing it. Its not keeping its suspension! If your wearing magnetic bracelets remove them before going near the bottle. If the solution is dark and in this state it should only be taken topically or as a mouth wash and spit job.
By boiling the water this makes conductivity much better. This also creates a larger particle size which isn't as good but if its stirred every 3-5 minutes its nothing major as we are talking about such minute figures anyway being Ionic. 20 minutes for 500ml is usually the time needed to 3ppm. I use an old Sota model also and I do the same except I run it for 40-60 minutes. You should get the clear solution as you stated earlier this is correct.
300ml is absolutely nothing my friend. Did you know with the solution you have been making it takes 1 litre of CIS to have an intake of 3mg thats all. I've took 300ml for 4 years sometimes 2 litres and no problems at all. If your still worried then drop to a 150ml range again. Sota's generator times are based on 500ml of water!