Is tea tree oil dangerous for my betta fish? What should I know?

Neem oil. An be toxic to fish if used in sufficient quantities. There are safer, more effective "natural" remedies. Melafix (contains melaleuca oil) or Pimafix (contains Bay Rum oil) are two products that can be used to treat infection in fish. Indian almond tree leaves can be used to reduce the pH of water. Plain aquarium salt can be used as a general tonic to reduce stress, improve gill function, treat minor infection and aid in recovery but some fish, especially scaleless species, are not salt tolerant, so care must be taken. The best way to keep fish healthy and to treat infection is to maintain pristine water quality via filtration and regular maintenance. Do not overfeed, limit light exposure and monitor temps and chemistry closely.

I would not put a turtle into a tank with a male betta, The betta has flowing fins which may look very tasty to the turtle. Depending on the breed of turtle, it may eat the fins off your fish or even the fish.

Tea tree oil is typically used to treat fungal infections, so it probably wouldn't have much effect on fleas or ticks and it can also be toxic if Holly licks the area you apply it. It would be better to use a topical designed for treating these, such as Frontline or Advantage, and would be much safer for Holly. You can also use a flea comb to remove any dead fleas after giving a bath, as well as make sure to clean the environment such as bedding, baseboards, etc to help prevent reinfestation.

I do not recommend the use of essential oils in the presence (ie the same room) of parakeets or any other birds. There are some oils that are toxic (such as tea tree oil) and others that may not be, however toxicity often depends on the concentration of the oil and while an oil may not causing toxic signs in one bird, it may in another more sensitive bird. There is no way to know other than trying it out, and , honestly, I would not risk my bird's health just to find out which oil does not harm him and which one does. If you use essential oils outside of Garcia's room, there should not be a problem.

Creek water would not be recommended as you would likely need to heat it and the risk of waterborne pathogens (even in fast moving water) would be greatly increased. Ordinary tap water can be "aged" by leaving it out in a wide mouthed container for 24-48 hours prior to use. A water conditioner would still be recommended to minimize stress after water changes.

Tea tree oil is toxic to dogs and should never be used on them unless significantly diluted. Tremors are a sign of poisoning, and even though Lil bit seems ok now, I would still have her seen by a vet right away for a health check.

Betta are definitively very territorial and you may notice multiple fights for space at the beginning. Definitively the aquarium must have multiple spots for hiding like Terra Cotta Flowerpots and PVC Piping, moss or bushy plants, ornaments, rocks and driftwoods. This will avoid open fights and will provide good places to have some rest.