It sounds like you are either under watering, or over watering. The first rule to grasp, is that when someone tells you a species is thirsty, or requires a lot of water, this may be true, but you have to put this into perspective, as this does not mean the tree wants water continuously, it just means you have to monitor the levels more often for when it does need a water.
Under watering, can often occur in the sense that the root ball is so compact, that water is only barely able to penetrate the edges of the root ball even after you have given the tree a long watering, and thus the main heart of the roots are not getting enough water. You would be able to see this during a repot. However, if you are leaving the roots to soak for an extended period, then by natural absorption, the water will fill all the nooks.
Over watering is the other most obvious candidate. The symptoms of over watering are similar to under watering, in the sense that the roots cannot breathe and thus cannot transport nutrients or water to the tree. Basically as the roots rot, the leaves start to wilt and discolour yes, but generally you can
usually tell if it is underwatered because the leaves will wilt and dry very rapidly, whereas overwatering takes a longer time to die.
I would not recommend having a water tray for callistemons as most varieties are susceptible to over watering or periods of inundation. Get your finger and push it in a really good depth into the soil to check the levels of moisture before you water each time. If its still damp, do not water. You cannot stick to a regime of watering that involves watering the trees at the same intervals each time. On a windy day, it will lost moisture quicker, and add sun to that and it is even quicker. On a damp cold lifeless cloudy day, it may not even need any water....
And if you are keen to read more, which is always recommended... I recommend reading the following post with a link attachment at bottom :
Re: keep moist, don't overwater?
Postby bonsaibuddyman » May 28th, 2014, 12:15 pm
The science of watering container potted plants, whether they are bonsai or not is essentially not something you can learn on a forum. Go to a nursery, and ask them to show you when is a good example of a potted tree that needs watering. This is the best advice you can give anyone, as you will see first hand what is required. Are you asking for yourself, or just in general, as with your bonsai age there, I am guessing you mean in general. You are not meant to keep the soil moist.
Step 1. Water bonsai thoroughly, and allow all the excess water to run free.
Step 2. Do not re water the bonsai until the soil is just about dry. (This can vary for your different types of trees, as some will use the water faster than others, so you cannot have a system of watering that is the same for all your bonsai. Hence, the horrible downside of owning bonsai. But apply Step 2 for all your bonsai. Otherwise, if you are just rewatering all of them when you think the majority of them need more water, then chances are that you have too many bonsai to look after.
Repeat steps 1 & 2 until you are due to enter a nursing home. In which case, step 3 is required, which you wont remember anyway, because you'll be old and senile.
viewtopic.php?f=7&t=18012 
They get Seasol every weekend and power feed as well...
Have not had enough experience as yet with them to give you a proper answer 
