Blue-green algae is a toxin that often looks like a paint spill in lakes, ponds, and streams. Blue-green algae is most often found in warm, shallow water and is harmful to humans and pets. Ingesting water contaminated with blue-green algae can poison your dog and cause symptoms like weakness, vomiting, diarrhea, and seizures. Call your vet immediately if you suspect your dog swam in or ingested contaminated water.