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Your description indicates that the sediment filter (sand trap?) is clogged. If it's a cartridge type you need to replace the cartridge. The diaphragm tank has nothing to do with this issue. The tanks only purpose is to prevent short cycling.
When a filter gets clogged it can significantly reduce the flow. That's when it needs t be replaced. The only other likely culprit is a broken main shut off. If someone closed the main valve at some point recently that too could restrict the flow if the valve broke.
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