Apple cider vinegar depletes oxygen which will inhibit algae growth and it will harm goldfish and koi. Goldfish and koi both need a lot of oxygen. Keeping goldfish and koi in an unfiltered livestock is a death sentence. A single koi needs to be in a 1,000 gallon filtered pond, and each additional koi needs 100 gallons of water. Common goldfish (single tailed variety) need 50 gallons of water per fish. All outdoor goldfish need a properly filtered and aerated pond. Mosquito fish are fine to keep in livestock tanks as they will consume any mosquito larvae present in the tank and will reduce mosquitos from hatching.