Just throwing it out their, heating the room is most likely more cost effective (unless you have to finish the space around the tank). HOWEVER, if you look at the below, the OP notes that he will be using 2 4500w heaters. 2 things you need to know tho, first is these run on 240v lines needing 30 amp breakers a piece. Depending on the state you live in, your comfortably with your panel, and the availability of breakers this may cost you a lot. The second is that he plans to use an arduino which would be fun project but for those who are unfamiliar with programming it is not feasible. Luckily you can replace the arduino functionality with a pre-built temp controller specifically I would target Bayite DC 12v Controller from the site that delivers things via drone. If you must use 120v you can use the same pieces but swap the heating element for 2k W 120v elements. Either 120 or 240 will require a dedicated breaker tho.
If you're going to heat it electrically, it might be worth looking at heatpump water heaters. More expensive up front costs, but would cut your power costs by 2/3rds or more and probably pay for itself in only a few years.