macro just dies?

So I just had a general question for you guys, nothing major or anything but I was just wondering.

So I have macro in my display and it grows really well, however after about a month of it being in there all of a sudden it turns brown and disolves. I've tried the feather macro and the macro with the balls all over it ha ( forget what that one is called).  The only one that doesn't just up and die is the chaeto but I'd rather keep one of the other two.

Any idea what is going on? or is this normal?

Once again thanks in advance for the help.

I have a 90gal tank with a 30gal sump

params are as follows:

temp goes from 78-82 through out the day
nitrate 10ppm
nitrite 0ppm
ammonia 0
ph 8.2
cal 410
haven't tested alk in about a week I ran out of the test solution so I have to go to the lfs today, this couldn't be a problem could it?

Jeff points out an important variable.
The water parameters you listed are some of the variables. I have not seen that kind temperature limitation for Caulerpa so there must be another variable at work in Jeff's tank that effects the Caulerpa when the temperature rises. Caulerpa has many varieties. One type will do well in one tank and not so well in another tank. The variety with the little round balls as fronds is often called Grape Caulerpa though I have seen at least 3 variations of Grape Caulerpa.

One of the most important growth variables is food. Are the nutrients that are available to the algae sufficient for it's needs?  We find that Chaetomorpha is a very aggressive nutrient consumer. It often wins the competition for food/nutrients. The key to keeping Caulerpa and Chaeto, along with other Macroalgae in the same Refugium, is twofold. I'll give a general guideline here:

1- Harvest the Chaeto when it grows to more than half the amount of Caulerpa in the Refugium
2- Position the Caulerpa upstream from the Chaeto

BTW, how fast does this tank use Alkalinity Keeping Alk and Ca levels fairly constant helps keep the tank healthy. Eventually, you may not need to test weekly. I test monthly or whenever I feel the need. Just add the amount of Alkalinity that the tank has been using in the past. Do you know that Baking Soda is the Alkalinity chemical additive

I dump a little bit of Iron supplement in my refugium every so often. I generally get great macro growth, but I also have a 225 and feed heavy w/ frequent water changes, so there is a lot of nutrients floating around already.

I was under the impression that temps that are too low cause die-backs and not the other way around. I would be suspect of your tank dipping into the 70's first and being oligotrophic (nutrient poor) second. But that depends on the age of the tank and the type of macro. After conditions in a tank stabilize (in about a year) it becomes more difficult to sustain caulerpra growth. Nutrients that are undetectable on test kits can still sustain massive growths of caulerpra which is probably the reason that it is undetectable in the first place. IMO being unable to keep caulerpa alive is a good indication that your tank is mature and stable. Also, as far as nutrient removal goes in most cases c. prolfera (blades) and c.racemosa (grapes) are much more effective than chaeto. A good way to figure this out is to just let your fuge run wild whichever species dominates over time is best suited to your system. Your fuge is an area in which you should not interfere with nature.