Instant erdication of all algae

Not exactly a last resort method - but close. You can bleash your plants and bleach kills every alage known.

Blue-green algae

Blue-green alge or Cyanobacter isn't anywhere close to regular green algae; it more resmbles bactiera from a biological standpoint and it one of the first life forms on earth. It's not tight to get rid of though.

Staghorn algae

Nasty blck stuff. Also called "hell algae". For a reason.