The Nautilus tanks in my own personal opinion are top tier in their class (mouth to lung vaping). I highly doubt it's the tank that's giving you the issues, and more so the juices you're vaping. Personally, I've found juices that had 12mg be so harsh that it almost felt like vaping pepper, while some felt like I was breathing in air. You really just have to do some experimenting with juices and find one that hits the spot, can be vaped for long periods of time without you getting sick of it and most importantly, is budget friendly. This is the beginning stage of vaping in my opinion, and perhaps one of the more costly stages.
My best advice I can give to you is to find something a lot of other people are vaping, online (local stores may be a business you want to support, but their mark up prices are almost always ridiculous), and not off brand juices no one has heard of. Personally, when I vaped higher nicotine juices using mouth to lung, I vaped the following juices and all of them really hit the spot for me.
Candy Cane from MtBakerVapor. This juice is a minty, but not overly minty juice with a good throat hit and a light sweet aftertaste of a candy cane. It's really refreshing and not overly powerful. Also great with a cup of coffee! I usually got 12MG nicotine, 50PG/50VG and 2 flavor shots. Juice is very inexpensive at this site, but it can be difficult to find a quality juice you enjoy due to the magnitude of quickly brought out flavors, simply to have flavor variety.
Honey Pearry from TheVaporChef. Very hard to describe the flavor, but it's like a honeydew melon mixed with pear. Probably one of my favorite juices of all time, that I can't vape anymore because I'm so tired of it. I stuck with this one for a good 5-6 months before I moved onto rebuilding and using higher VG juices. Again, 50/50 blend if you're going to use the Nautilus.
Hopefully you can find a juice you enjoy that offers the right amount of throat hit to satisfy you. I'm very picky, so I had a hell of a time.