30 gallon breeder tanks are usually 36" long by 18" deep by 12" high
There is also a 29 gallon tank that is 30" long by 12" deep by 18" high. A 30 gallon breeder tank size would be perfectly fine for a single dwarf
If your tank is more like the 29 gallon one - ie 30" long by 12" deep - then it's probably fine if they are happy in their tanks. 12" is a little narrow for the depth but if you have lots of depth of bedding and lots of toys etc for enrichment it can be fine. Lots of people used to use Clearseal tanks for a dwarf hammy and they were only 12" deep. Having another level in it would help as well - eg a platform on legs. If you could upgrade them to 30 gallon breeder tanks it would be better - but if you don't have the space then adding enrichment will help - and the best enrichment s plenty of depth of bedding and lots of hidey places (eg little cardboard boxes and toilet roll inner tubes etc and a platform to sit under/on. And not cleaning them out too often so their tunnels last a good length of time