sinkend
falling
Decreasing in value, amount, intensity, or quality.
Die Firma leidet unter sinkenden Verkaufszahlen.
The company is suffering from falling sales figures.

falling / sinking
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falling
Decreasing in value, amount, intensity, or quality.
Die Firma leidet unter sinkenden Verkaufszahlen.
The company is suffering from falling sales figures.

'Sinkend' (falling/sinking) is the present participle of the verb 'sinken' (to sink/fall). It is used like an adjective and always describes an ongoing process or continuing action.
Die sinkende Nachfrage macht der Branche Sorgen.
Falling demand is worrying the industry.
As an adjective, 'sinkend' (falling/sinking) usually stands directly before a noun and is declined accordingly. It describes the noun more closely.
Trotz sinkender Temperaturen gingen die Kinder nach draußen.
Despite falling temperatures, the children went outside.
In the meaning of 'abnehmend' (decreasing), 'sinkend' (falling) is often combined with abstract terms. Common examples are falling prices, falling interest, or declining quality.
Die Partei kämpft mit sinkenden Umfragewerten.
The party is struggling with falling poll numbers.
When it is about downward movement, 'sinkend' (sinking) is often used with objects that change their height. Typical examples are a sinking airplane or the sinking sun on the horizon.
Die Passagiere des sinkenden Flugzeugs konnten gerettet werden.
The passengers of the sinking airplane were able to be rescued.
increasingly / waxing
'zunehmend' and 'sinkend' both describe a change over time. The difference is the direction: with 'zunehmend', something becomes more, stronger, or more frequent. With 'sinkend', something becomes less, weaker, or lower. 'zunehmend' is often used as an adverb, for example with problems, interest, or costs. 'sinkend' fits especially well with numbers, prices, values, or quality.

Wegen der sinkenden Temperaturen bleiben die Kinder heute drinnen.
Because of the falling temperatures, the children are staying inside today.
Viele Menschen arbeiten zunehmend von zu Hause.
Many people are increasingly working from home.