Welcome to PB.
The reference to making "the water so toxic" is a gross misnomer. The principle behind using herbicides for the management of aquatic vegetation is to target the offending plant(s) with little or no collateral impact. I'll skip a treatise on the subject since that wasn't the purpose of your post.
Back to your question. I'm not intimately familiar with yellow floating heart (which is an exotic invasive species), but I've dealt with many of its cousins.
Clearcast or Imazapyr are relatively effective treatments for most of the floating heart species. However,
eradication may not be possible once this genus becomes thoroughly established and entrenched; which means that "management", through multiple means (including herbicide treatments at regular intervals), may be your only viable option.
