Phalaenopsis bellina - leaf tips darkening, turning yellow, spreading down leaf

[OzPhal]

Hi Everybody,

I was hoping to get some advice on the following:

Plant: Phalaenopsis bellina seedling 6 leaves so it is i would probably say 12-18mths old
Growth Conditions: Semi-Hydroponic, 25oC - 30oC, 70%-90% humidit, grown in a covered aquarium (with other seedlings), has an aquarium heater in the reservoir to keep the water at 25oC along with a large fan for air-movement. The whole setup sits in front of a south east (Australia) facing window with lace curtain in front of it. I have fertilised in weak concentration with Growth Technology "Orchid Focus"

I have this seedling sitting in its own independent reservoir within the aquarium. There is some algae growing in the reservoir. What i have noticed is that the older leaves, one at a time, start appearing thinner at the leaf tip, then the tip goes blackish and dry. Then yellowing starts to move down the leaf gets to about halfway. To date only one leaf has been effected in this way - i looked over the plant and gently moved the leaf and it fell off in my hand. The base of the leaf was yellowed. A second leaf has started to show the blackening tip with the descending yellowing.

Any advice? The plant is isolated in regard water supply although it is in a common growth area as other seedlings. I don't want to lost this seedling as Phal bellina is one of my favourites