why cant i keep macro algae alive?

[am3gross]

why does my macro algae die? i cant seem to keep this stuff alive. i have a 75 gallon display and a 29 gallonish fuge. i have tried the calurpa and the cheato and neither are doing well at all. at first when i added the calurps it survived for about 6 months. then ai added the cheato and the calurpa started dieing. i thought they were competing for nutrients so i was ok eith just having the cheato. but now the cheato is dieing. does anybody know what is up? all help is appreciated
am3gross

 

[emperor11]

Do you ever harvest the algae? You need to do this every so often, to allow new to grow.

 

[reefnut]

Ditto on harvesting it and what lighting is it under??

 

[am3gross]

no i never tried to harvest..... the lighting is a 10 k probably a 18 watt maybe sump light and a 55 watt 50/50 bulb. lighting should be good...... i think anyway.... i also have 3 mangroves.... is it true that they might be fighting for nutrients and the mangroves are winning?i also have about 20 pounds of live rubble in the fuge.