If the probe is hanging in the air, it is measuring ambient temperature. Ambient temps should be between 85-90F. The basking temperature should be between 100-120F (110-115F is a decent temp to aim for...I think a little hotter is alright, as long as there are cooler areas of the basking spot a dragon can choose from).
To measure basking temperature, place the probe directly ontop of the basking area. MOst digital thermometers need 10-15 minutes or longer to measure current temperatures...they can be rather slow to register changes hence the need to leave the probe at a certain spot for some time when its moved. The instructions/information usually packaged with the thermometer should indicate the 'stand' time needed to accurately measure a temperature at a new location.
Temperature (temp) guns can be found at automobile supply stores (their original use is measuring temperatures of engine parts) or you can purchased them online at Pro Exotics ( www.proexotics.com ). These are much more accurate and you don't need to wait at all to find out the surface temperature of any given location, just point and shoot.
As the dragon is 5 years old, its done its growing, at least length wise...it can gain weight and fill out or get overweight if fed way too much (not a concern with yours, since from what you mentioned earlier, it is underweight). Crickets fed should be no bigger than the space between its eyes...so a 5 year old dragon of normal growth, should have no problem eating large crickets. If it is as small as a hatchling or juvenile (14" or less) it is really undersized...unless its not a pure inland bearded dragon or a Lawson's bearded dragon. If you can get a picture of it, especially from above, we will be able to tell if its a Lawsons' dragon. (or Rankins). These dragons don't typically get much bigger than 12-14" in length. Regular Inland Bearded dragons range between 16-24" with 18-20" being average).
At the vet, definitely get its weight and total length, (snout to tail tip).
Newspaper is fine as a substrate as well. It may also be causing the 'darkness' on the belly though...especially if its pretty uniform (if more like bars, than that is natural pigment.) As newspaper ink easily comes off, especially if the dragon is damp from misting or being in water, it will turn a dragon's belly and feet black in time. I personally feel unprinted newsprint is better, as you don't have the mess from ink used on newspaper (also in some areas, the ink can be toxic though most publishers use soy based inks. Colored inks are often still toxic even if the black ink is soy based). Regular paper towel works fine too.
To use the probiotic, the bottle of acyliphis should include instructions on how to administer. I haven't used it myself at all, so can't say how much to use or how it should be administered. Its definitely administered orally. However, if you are feeding 'Critical Care' to your dragon, it may not be needed...check the information with it, see if it includes a probiotic or 'appetite stimulant' Maybe BRLvr can shed more light as he's used this product before with his rescues.
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PHLdyPayne
Forum Princess