DONT TRUST LOCAL FISH STORE

[Saltyreef]

It could. My mother took care of my tank when I went out of town, not knowing my phyto culture was dead, and because I had her dose it regularly, I returned to see a whole ton of algae which potentially could destroy a tank if it is bad enough.

over run by algae is far from killing a tank though.
your issue sounds like the tank suffered from improper nutrient export and lack of husbandry while you were gone.

Live phyto can contribute to a reduction of nutrients and a properly sized skimmer will take care of any excess dead phyto. Also a proper refugium will account for any excess phosphate built up from the dead phyto that isnt consumed, skimmed or filtered before phosphate has a chance to build up to the levels beyond what the liverock and substrate can handle/hold.

The nutrient export should be sufficent enough to handle the normal bioload as well as any small extra build up from uneaten food and the occasional death.

Some people even find the need to dose nutrients to keep their levels detectable to satisfy some types of coral when they arent heavily feeding or don't have a sufficent bioload and a higher level of nutrient export.
It is a delicate balance but there should be a baseline for everyones specific situation.

 

[Tinnerito]

that’s a stretch. If you threw the whole bottle in, dead phyto could cause some problems. But kill a tank? Show me evidence that it’s happened

Got me there

 

[Science/G]

Use Reef Roids instead.

 

[Tinnerito]

Use Reef Roids instead.

Phytoplankton has benefits that reef roids don't. I personally use both.