I'm not saying just give it to us, but make it an unlockable.
15 Almerian Seals - 150 Mist Capacity Tank
20 Almerian Seals - 200 Mist Capacity Tank
30 Almerian Seals - 250 Mist Capacity Tank
50 Almerian Seals - 300 Mist Capacity Tank
Make the item a recipe that binds to the player on pickup, instead of a sudden increase to Mist Capacity. Players need to lower level Mist Tank to craft the higher level Mist Tanks. The tanks also bind on pickup, so they cant be sold or traded.
It doesnt have to be Almerian Seals, but those are the ones that hard are to get, so players will need to earn the ability to accumulate more Mist. You could even add in Mist tokens that randomly drop from bosses or rare chests, make them even harder to acquire. And add a mist merchant! More tall creepy dudes hanging out in Haven!
The other idea is to make a Mist Set of gear. Armor and weapons that increase your Mist Capacity by 20 for each piece equipped. Make them hard to find rare recipes, and the gear itself wouldn't be BAD, but it would be less powerful. Taking off the Mist Gear lowers your mist total, so if you have 5 pieces equipped to login with 100 extra mist, then take it off to put up your leveling gear, your capacity would drop and you would waste any mist of your current capacity. Basically its a choice between powerful gear, or decent gear with more energy.
Those are some very interesting suggestions.
I don't think that the mist tank should ever get as high as 300, as that just seems like too much: it would enable people to craft a 4* item AND do a whole tier run on one tank, which is a lot. However, the rate of ME recharge wouldn't change, so 300 ME would still take 3 days to recharge...but players would only be logging in every three days.
There are a number of ways to make your suggestion work, I think.
I don't think the tanks should be bought with Almerian Seals; there are some players who already have more than 50, or will have more than 50 very soon. There needs to be another way for the extra tank to be obtained.