Fountain Pens

Seconding Aurora Borealis which has some great shading. Almost always have it inked up in a broader nib or stub.

If you like the Iroshizuku inks that you have, I also like Ku-Jaku, which is maybe on the bluer side of Syo-Ro.

If you want a different brand, I have really liked many of the Robert Oster inks, and they make a very nice looking teal with good shading called Deep Sea.

When in doubt, you could always order some samples and try them out! I personally usually get samples from Goulet, but there are other options too. For Diamine inks, cultpens periodically has good sales (both are UK based so usually much cheaper than US retailers anyway if you're in the USA) and it can be pretty affordable to pick up a bunch of smaller (30mL) bottles to try out while only paying one shipping fee.

I'm a fan of Colorverse Gravity Wave

Diamine Aurora Borealis is a beautiful teal. Celadon Cat is also worth considering if you like shading, although it's much paler.

I'm also a huge fan of Diamine's Aurora Borealis! It's a lovely ink that I use year round.

Thanks for all the information, everyone! I guess I'm gonna have to get a bunch of samples and report back, this seems like it will be fun!

Diamine Marine is pretty nice as well.

I have just ordered Diamine Eau de Nil - maybe that could be something? Haven't received it yet, so can't say much about it. But I really enjoy my Tsuki-Yo! 😎

If you ever need a waterproof ink, check out Sailor Seiboku. It's described as blue-black but looks more like a dark teal. It's the only bottle of ink I've bought more than once—I like trying different inks—but that's because I need something that's more permanent for work.

Sailor Yama-dori is pretty, but very expensive.