You may find that the hydramat clogs up quite quickly in a semi-rusty tank like that. Off-roaders have had troubles with that, even using very large mats.
If it clogs up once, you've shot any chance of "saving money" going this route.
lol,,, Guys its NOT RUST! While it does't look that appealing, its not rust, not by a long shot. Its actually a healthy spray of some sort anti corrosion treatment. You can actually see the "runs" of the excess spray on application in the first pic. The tank is pretty clean.
Also Hydramats are 15micron so won't easily have issues with common tank debris.