If it does go sexual, its a quick road to meltdown.
SH will hate me for this, but I have to argue here too. I find the notion that macro is some sort of an inevitable tank-nuker is a bit like viewing a fish dying in the tank as an inevitable tank-nuker, but nobody avoids putting fish in because of that. Instead, they see the dead fish and remove it rather than letting it sit to rot and nuke the tank. Same thing applies with macro. Even when in my 12g I had most of my feather caulerpa bleed out, go sexual, and turn the water opaque green due to my Turbos having a one-time strange feast on the main vascular structures, I saw no significant effect on the water except that it was ugly for 2 days while the muck was filtered out. PH and all other levels showed no change during that time. This was also a very large volume of macro...once removed, it was several cups worth of squashed and deflated Caulerpa, so it was no trivial amount for a small tank. However, I had previously prepared my tank for such an event by keeping good surface aggitation, having additional oxygentation, having a higher than normal turnover, and once I saw the problem I did a small water change to try to increase visibility and removed the dead macro. As far as post-event-management went, it's pretty much the same thing I'd do if I had a decent-sized animal die.
Where tanks can really run into problems is if you have barely sufficient oxygenation in the water and/or not enough circulation. That situation poses a problem for phyto blooms too - when stuff starts to die off or be decomposed, the O2 gets sucked up really fast if it's not abundant and everybody's gasping at the surface all of a sudden. Another possible complication can be pH, which can drop under similar circumstances. After having some experience with the fallout from repeated phyto blooms in another tank (many times worse IME), the solution is pretty simple and works for macro issues too: backup oxygenation, e.g. an air pump in addition powerhead aggitation. A small water change is a good idea once you observe the problem (just like taking a dead fish out instead of leaving it to rot), but if you prepare your tank well it shouldn't crash and burn just because you wern't there to catch it before the tank turned green. I've never done more than a 10% water change on the few macro occasions and on the more numerous phyto occasions, it's never been at the exact time of the event, and it's never led to a meltdown.
