The quick answer: I don't think neem is safe to use in the restricted, closed environment of a vivarium or paludarium.
The details: Unfortunately neem is not safe for fish and other aquatic organisms. That doesn't mean you can't use it if fish or frogs are present. It depends on the specific use.
Using it in a vivarium/paludarium would not be safe in my opinion. I don't know what exactly you want to do, so maybe read through the details below and draw your own conclusions.
Neem extracts are not safe for fish. These studies have more information about the exact concentrations that are harmful. (Note that the neem extracts were added directly to the water.)
Toxicity of neem pesticides on a fresh water loach...: "The fish under toxicity stress suffered several abnormalities such as erratic and rapid movement, body imbalance and surface floating responding proportionately to the increase in concentrations of the toxicant biopesticides."
Acute lethal and sublethal effects of neem leaf extract on the neotropical freshwater fish...: "Fish exposed to all neem extract concentrations exhibited damaged gill and kidney tissue. These results indicate that although neem extract is less toxic to P. lineatus than other synthetic insecticides used in fish-farming it does cause functional and morphological changes in this fish species."
Other studies looked at a range of aquatic organisms, such as mosquito larvae and tadpoles. In some studies the neem compounds were toxic to all tested species, others reported that some organisms were unaffected.
There have also been some studies looking at the environmental effect of agricultural/forestry use of neem sprays on neighbouring water bodies. Here is one that concludes that it does not pose any problems:
Ecological risk assessment of neem-based pesticides: "The aquatic risk assessment process showed that the risk values of tested pesticides did not exceed the criteria, and therefore, no ecological hazard is likely to result from their use."
Ok, have I confused you enough?
What I concluded after reading through a couple of dozen of those studies is this:
Don't use neem in water. Use on land is fine.
The active compounds in neem are not stable in the environment, not at all. So unless you spray directly into a body of water, you should be fine. Agricultural use is fine. Take care near small, shallow and standing bodies of water.
Use in vivariums/paludariums? I would not risk it. No matter how you apply the neem product, chances are it will end up in the water.
Longterm effects of Neem on fish by: Anonymous
Some of the comments indicate that Neem is harmful to fish. I know that Pyrethrins, though natural and less toxic to vertebrates, are deadly to fish when they wash from land and/or run-off to rivers, ponds, oceans etc. Is Neem in this category, or does it break down more quickly than pyrethrins?
Neem breaks down quickly by: Birgit
Neem breaks down very quickly when in contact with moisture and under UV light. There is not as much danger from it being washed into streams, unless it happens straight away.
It shouldn't be applied right near water where it could enter the water immediately, or where overspray may hit the water etc., and I certainly would not recommend using it in a confined, moist environment like a vivarium. But during normal use outside there is nothing to worry about.
Spraying NEEM in a garden where frogs are present by: SJ
In light of everything said above, I suppose I'm still not clear on whether or not NEEM would be toxic to amphibians in my given scenario... I have a small balcony garden and two tree frogs have decided to live there...unfortunately, my plants have been riddled with this whitefly pest and I have to spray with something a little more effective than the safer soap I've been using. How will the NEEM affect the frogs? Thoughts? Thanks.
Little frogs in back yard by: Linnea
This time of year, early Fall, I'd like to spray the whole patio, raised vege beds and grassy wet areas...but there are numerous, delightful small frogs I am concerned about. I prefer Neem Oil. The soapy sprays I have used are only useful to a point.
Affect on amphibians neem by: Anonymous
I am looking to use neem oil on mosquito larvae in a small pond with no tadpoles present, however I am afraid that after I use neem on the pond, frogs will lay their eggs there, and if that happens there's no telling how much neem will be left, and from what I have heard, the only harmless creature it hurts is fish, and since it's a confined frog pond, there are no fish. What I want to know is how frogs and tadpoles are affected by neem. I need answers fast, because it will not be long before they hatch.
neem use in water by: Birgit
If you scroll back up, I linked to a bunch of studies in the answer to the originasl question, in the first and second comment. That is what is known about the issue.
The comment also had my conclusion: Don't use neem in water.
In my opinion there is a good chance it will affect the eggs.
How do I get rid n of fungus knats naturally... pets and fish present in terrarium by: Anonymous
How do I get rid of fungus knats with frogs and fish present in a terrarium? I'm so confused. Vaseline is not good for water nor frogs, neem cake was my last resort. I need help. Infestation is currently present, noticing a cluster in the back corner. Also, springtales present as well. Hypoaspis miles maybe but they eat springtales... f%u straying :(
Plant in fish bowl by: Linda
I have a plant, elephant ear, in a fish bowl with a Betta. I have spider mites on the leaves. I need to treat the plant with Neem. I removed the plant and saturated the leaves and part of the stems, not all the way to the roots.
Once this is dry, is it safe to put back in the fish bowl? What can I use to clean off the stem area to remove the dried Neem if it is not safe to put back in after dry?
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Neem should not be taken by anyone who is pregnant or trying to conceive. Neem contains compounds similar to those in aspirin and must never be used to treat children with fevers. It should not be taken by people with known allergies to aspirin and aspirin like substances. Neem oil should not be taken internally.