Millipede Humidity

[sorrybugs]

So I’ve had a new vivarium going for a few months now and I can’t seem to keep the humidityconsistent.
First question is 85% humidity too high?
Second: the substrate is far from dry. It doesn’t even encroach dry. Any excess water drains to the drainage layer. Should I be worried about it being too wet?

Spraying is generally mostly for the humidity needs of the plants but I spray in average every other day to every three days and have a heating pad stuck to the upper corner of one side of the tank and led lights going 8 hours a day (for the plants). Temp is usually around 75°F unless it gets super hot in the middle of the day (closer to 81°F) or drops at night (71°F).
Any ideas on how I can bring down the humidity if it’s harmful without simply removing all or part of acrylic cover to add ventilation (it causes humidity to drop 30% if I do that)

About my vivarium:
I have five ivory millipedes and one Philippine blue millipede living in a 20 gallon tank (yes I’m aware it’s quite large) with foam background on the back and sides halfway down. I just added more plants hoping to create a more stable environment (pretty sure the fluctuation caused one of my Philippine blue millipedes to die unfortunately). The substrate stays moist and is made of pasteurized top soil (perlite free organic with wood shavings) and collected rotting wood and lots of leaves mixed in and on top. I have a drainage layer of exoterra lava rocks that stays mostly full (I wasn’t able to make it as deep as I would have liked when designing the tank). I have multiple kinds of springtails keeping the bacteria and mold at bay. I have an acrylic cover to keep in humidity but when it’s covered the humidity sits solidly at 85%. Everywhere I look it seems like the cap is 80% humidity. Thoughts? Is it harmful for it to be 5% higher than recommended (this is generally the air humidity)?

 

[hamfist]

Firstly, remember that substrate moisture is a very different thing to air humidity - depending on various factors such as heating, ventilation, watering/misting/spraying technique, subsrate composition, amounts of leaf covering on substrate, local climate etc etc.
My personal general techniques have a tendency to let humidity stay high but the substrate can have a tendency to dryout, so I know I have to watch that. You will get to know your own enclosures and how they tend to behave.

Many folks don't bother measuring air humidity at all because its such a crap shoot in terms of being a useful figure. The MOST important thing is to keep the substrate moist to the touch below the surface layers. Most folks, including me, seem to just carefully stick a finger in and see what it feels like. This is rather subjective and I suspect you'll not get two people to agree on exactly what feels perfect but its probably the best way to go about it. A few species seem to be prone to rot problems if its too wet.
OK, the next thing to consider is air humidity, which I suspect the millipedes are generally less bothered with. HOWEVER .... you will tend to see a lot more of some species on the surface if you keep the upper substrate layers moist and the air nice and humid.
For me I like to see my millipedes, so I try and keep it all nice and moist and deal with the negatives that come with that - currently thats mites - oh, and always fungus gnats ! My system is to have a hygrometer about 1-2cm above the substrate, and I'm looking for humidity around 95-97%. I'm more that happy with 99% for a dy or two and if it drops below 95% I know my substrate is getting dry. But thats my system, with my substrate mix, with my heating and watering methods. Its all very personal, but I assume it could be a guide to someone else.
I state where exactly my hygrometer is because that makes a huge difference. I know the reading will be 10% or so less than that a few cm higher. So I have a standard place in all my tanks. The variation due to height off substrate will be more the greater the ventilation of the enclosure is.

I wouldn't worry too much about consistency of humidity for most species. If it gets a bit dry for them on the surface they will simply bury themselves until it gets a bit moister.

You ask ... can the substrate get too wet ? and the answer has to be yes, it is possible for the lowest levels to get waterlogged and rather anaerobic. Thats going to start to be toxic to your whole system. Some folks like a drainage layer. Personally I don't use them, so i can't advise on that. I'd say if you have any sort of waterlogging in the drainage layer then that probably needs to be sorted and removed somehow.

As for lowering humidity then your friend is ventilation, and one sometimes has to be a bit creative. I personally have a fine mesh cover over the complete top of the aquarium and then have clear acrylic sheets that I can cover as much or as little of that as I want. Folks use a million and one different strategies to control ventilation, because we all use such different enclosures and lids. So .... be creative. For me, I'd say its fairly important to have some way of adjusting the ventilation. We all over or under water at times and need ways to compensate, and ventilation is a main strategy that one can use.

I know I have rambled a bit, I hope it has some use to you.