Hello there,
Because undergravel filters are substrate-based, they pose problems for a planted tank. Having such a filter means having to regularly vaccumm the substrate, which isn't good for plants. Moreover, plant roots will grow all over it, eventually choking it up.
Another reason will be most equipment ( C02 reactors, UV sterilizers, blah blah ) are best connected to a cannistor filter. UGFs, frankly, are already/going to be obsolete.
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