Great thread and a great shallow one you got there! I was thinking of doing the exact same setup! I have 5 seedlings right now; one in the sump, one in the display and 3 in the overflow temporarily (long temporarily

).
I've had my mangies since December, red mangroves Rhizpora Mangle. Got them off ebay
At first, they were very slow to show any growth, but then I started spraying them with RO water at least once a day and BAM amazing growth! Maybe you should try that!
Also temperatures here in Egypt have been soaring all summer, ambient is around 30-32 degrees C and water temperature aroung 28-29 degrees C (sorry, no fehrenheit

) , which is too hot for comfort I know! But no chiller and no AC

Also somewhat humid as I live in a coastal city. Maybe this weather helps them feel at home?
We do have natural mangrove forests here in Egypt (although a different species I think) and they live in super hot areas near the southern end on the Red Sea where day temperatures can reach near 40 degrees C or more with blasting sun all year round.
Keep up the great work and inspire us! I hope I helped
All the best!