I'm gonna reach out to a solar off-grid guru and tell him what I plan to do with my aquarium and see what he thinks will last the longest and what voltage I need and how many batteries I need.Another thing to keep in mind for using a LiFePO4 to make your own battery backup for the ecotech stuff is that someone posted the battery and it is listed as 18AH. On 100AH battle born battery would get more than 5x the run time as an echotech battery even if you assume you can use 100% of that 18AH. That battery is $900 and you would have to add adapters and a charger so about $1k. Bulk reef supply has the Ecotech battery at $173 and you would have to get 6, maybe more, to match the run time which brings you to $1k.
Basically, they come to about the same price. One is purpose built and looks ok, the other would be hobbled together and may be a challenge to make look good. One solution you have multiple that would have to fail, but they don't seem to last all that long from what I have seen. The other you would have only one that would have to fail, but it should last you a long time.
Relion does have smaller batteries, so you could go the LiFePO4 route with multiple smaller batteries for more redundancy or lower cost. I had emailed them when I was thinking about doing this and they said that the LiFePO4 chemistry does not degrade when held at 100% charge. I would still like to see some evidence of this personally.
Lithium Iron Phosphate is the way to go IMO. I'm gonna hook mine up where it will automatically come on when the power shuts off. I may not even have to fire up my generator for 3-4 days.