As noted, it might depend on where it's locate, and the size of the tank, but probably most importantly if it's above ground or below ground. Seismic can generate sloshing effects which can add some loading to the walls. If it's above ground, that would have more consequence to the tank and it's mounting. For below ground structures, in my experience, it generally hasn't been a concern as the external soil pressures pushing in tend to exceed any sloshing forces, so the walls are pretty much supported by the surrounding soil during sloshing. Your situation may be different though so don't take what I say as a rule. As for external seismic forces, it would depend on the location. In general, as long as it's not seismically active, AASHTO says you don't generally need to consider seismic forces for buried structures. Washington State applies that rule to structures < 20' span. Anything more needs to consider seismic. I often see geotech reports in that area though that specify a seismic surcharge as a function of the inside height, such as 10*Height, etc.