Hi Rich,
There are a few things to consider when evaluating an underweight axie. Firstly establish if the axie has a good appetite or inappetant. Also, define whether the axie is capable of consuming the food. They might appear to feed but not swallow anything.
1 - Check water parameters and temperature. Make sure you have 0 Ammonia, nitrites and <60 nitrates and an ideally neutral pH of around 7. Temperature ideally around 16-18 degree celsius.
2 - Are there other signs or illness or stress?
3 - What is the faecal matter like? - well formed? black tarry?
4 - Has there been frequent regurgitation? Water below 10 degree celsius can increase regurgitation. Tainted, decomposing food or undigestible food can also cause the same problem.
5 - Has there been evidence of parasitism? Especially internal parasites.
6 - Is the mouth slightly agape all the time? Think stomatitis, an inflammation of the oral mucosa. Axies may appear to feed, but can't swallow.
If your axie is feeding properly but losing weight consider the following.
1 - The food source. What types of food, in what amounts and in what frequency are you feeding the axie? Ideally feed a staple of worms and pellets.
2 - Parasites. Helminths or gut parasites can extract nutrients from the axies ingested food and also feed on the axie's body - blood, plasma etc.
3 - Protein losing enteropathy. - Sometimes some illnesses can cause the axie to excrete nutirtion without being properly digested. Normally the stools will look unusual.
Do not try to overfeed your axie especially if it has been emaciated for a while. This can cause metabolic disorders (much like refeeding syndrome). The gut and liver will not be able to be cope with the sudden influx of food introduced and can get damaged in the process. Sudden huge volume refeeding can KILL your axie. They can also get a form of bloat as a result.
Cheers