FredL

From the looks of your tree, it looks barely possible that you might save it. You will definitely have to make some changes in how you are caring for it, however

1. Soil. There are many successful varitions of bonsai soil mixes. They all have certain things in common, however. First they are granular and fast draining. Second, they retain water within the granules but not between them. I use a mix of about 1/2 crushed lava rock or other hard gravel, about 1/3 bark or wood based mulch and about 1/6 garden loam. There are lots of places to get the ideas of others regarding soil mixes. If most of the water you add does not drain out in 10 minutes or so, you need a more granular mix. If the soil mix dries out entirely in 24 hours, you probably need to add components, such as mulh, that retain water better. From your comments, I don't have a clue as to whether your soil mix is contributing to the problem.

2. Watering. Most enthusiasts water using a watering can or hose with sprinkler from above. I water daily. Every 3 or 4 days in the Winter, unless we have an extended cold spell. My impression is that regular watering from above is much better than infrequent heavy soaking. You should be aware that roots need air just as much as they need water. This is why a soil mix that has lots of space for air between particles but retains water within particles is optimal for roots.

3. Air/light/temperature. These three items are, without question, major contributors to your problem. Bonsai are, basically, outdoor plants. Unless you are working with tropicals specifically adapted to the conditions you describe, expect your trees to go into decline and eventually die unless you have gone to great lengths to adapt your indoor environment to duplicate outdoor conditions. Especially, trees from the temperate zone require Winter cold to live successfully. Temperate trees are adapted to go through a period of rest during the Winter months. They can not adjust to the year around tropical conditions inside a house.

4. Fertilizer. Probably not a problem; likely even beneficial. I'm not familiar with the specific fertilizer you're using but it sounds like you are using it appropriately.

Hope this is helpful. The biggest contributor to your problem, at least imho, is the indoor "climate" you are subjecting it to.

Best regards, Fred