The microtubule-associated protein tau promotes the stabilization of the axonal cytoskeleton in neurons. In several neurodegenerative diseases, such as Alzheimer’s disease, tau has been found to dissociate from microtubules, leading to the formation of pathological aggregates that display an amyloid fibril-like structure. Recent structural studies have shown that the tau filaments isolated from different neurodegenerative disorders have structurally distinct fibril cores that are specific to the disease. These “strains” of tau fibrils appear to propagate between neurons in a prion-like fashion that maintains their initial template structure. In addition, the strains isolated from diseased tissue appear to have structures that are different from those made by the most commonly used in vitro modeling inducer molecule, heparin. The structural differences among strains in different diseases and in vitro-induced tau fibrils may contribute to recent failures in clinical trials of compounds designed to target tau pathology. This study identifies an isoquinoline compound (ANTC-15) isolated from the fungus Aspergillus nidulans that can both inhibit filaments induced by arachidonic acid (ARA) and disassemble preformed ARA fibrils. When compared to a tau aggregation inhibitor currently in clinical trials (LMTX, LMTM, or TRx0237), ANTC-15 and LMTX were found to have opposing inducer-specific activities against ARA and heparin in vitro-induced tau filaments. These findings may help explain the disappointing results in translating potent preclinical inhibitor candidates to successful clinical treatments.