Since this is an eyedropper pen, to fill you have to unscrew the nib and grip and fill the pen that way. The body of the pen holds a whooping 4ml and it will last you a good long time. But this isn’t simply an eyedropper pen, it has a small seal attached to a piston mechanism to seal the ink chamber away from the feed. That makes flying with the Opus 88 Koloro Demonstrator is way safer than a regular eyedropper pen. But when you are ready to use it, unscrew the knob at the end. You just need it about 2mm of space for regular use.
Then the nib, I opted for a fine nib which in hindsight is a bad choice. The steel nib writes ok, nothing special but it is dry, very dry. And pairing a dry fine nib with a 4ml ink chamber seems like a bad idea after I’ve used it for a while.
Now, comes the cleaning. I don’t usually cover cleaning but this pen has frustrated me with keeping it clean. This is a demonstrator pen so the hope is I’d use inks that do not stain. I’ve filled it with Sailor Nioi Sumire and before that Monteverde Moonstone. Both of them stained to a certain degree and it wasn’t easy to get all the ink out even though it is an eyedropper pen. There is ink clinging to the inside of the pen barrel and cap for no reason I can figure out. Washing it with an ultrasonic cleaner helped some but there are still ink stains that just won’t go. The stains only got removed by soaking it in pen flush. The pen flush worked like a miracle but I don’t want to resort to a pen flush to clean this pen every single time.
Overall the Opus 88 Koloro Demonstrator is a value for money fountainpen but the pen is just not for me. The larger size doesn’t fit my hand well, the nib coupled with the large ink capacity is also not working well for me. The nail in the coffin for me is how hard it is to keep the pen stain free. The pen is entirely serviceable but it is just not for me.