Rumor says that you should rotate your fountain pen ink every 6 months to 1 year. This is another urban legend and is completely false. As long as the bottle stays tightly capped when not in use, the ink inside can remain usable for a very long time. Months at least. Probably years. The only enemy to stored ink is evaporation, and a tightly capped glass bottle prevents that.
Some heavily pigmented inks may look a bit muddy after a time on the shelf. Gently mix the pigment back into the ink by swirling it around in the bottle while the bottle is capped. Don’t shake it about… it isn’t necessary.
And, Greg Clark (author of “Fountain Pen Inks – A Sampler”) adds, “I would suggest adding (avoiding ñ ed.) sunlight. Don’t store bottles of ink for a long time in bright light or for any time on a sunny window ledge. The dyes fade – badly in some cases, like with turquoise inks.”
If the ink seems a bit thick, use an eyedropper and add a drop of plain water to the ink. Swish it about, then fill a pen with it and try it. If it seems ok, you’ve fixed your problem. It really is that simple.