I use both UGF and a typical hang on the back filter with those biobags... I always thought you needed both in combination... Back then when we got our tanks they came with with both... One is a 22 year old 30 long gallon tank, the other is a 14 year old 27g hex... Both have the original equipment.. undergravel filter with the power head and the hang on the side type filter with the biobags...
I just started getting into plants.. I have typical gravel, and its above 2-3 inchs deep.. I have a large anubius, very large amazon sword, and I forgot the names but I have a pinkish purle stringy type of plant in the tanks, and I have red nesaea which people say are super hard to take care of, and I have money wart..
Honestly... I think people who complain about undergravel filters are just noobs and don't know how to take care of their tanks..
I have never had a huge ammonia, or nitrate or anything spike in my tanks before and never had any issues with either tank using them... Same goes with people who say gravel isn't a good substrate. That may be true but my plants have been in my tanks for about 2 weeks now and none have died.. none are turning yellow, and in fact those red nesaea plants I have which people say are almost impossible to take care of or keep red.. Well they are still blood red for me... Some are slightly greenish yellow but they were like that when I got them...
The nesaea and money wart and all my other plants are sprouting roots, and I forgot what its called but they are growing roots outside the stems in the gravel as well. They are all maintining there color... Im using a lot of ferts tho.. I put in about 4 root tabs around my plants in each tank, but I noticed those just made the water extremely cloudy for 1-2 days I think they dissolved in the water, even tho I buried them I think the undergravel filters dissolved them... But im also using API leaf zone for iron and potassium, and im using API CO2 liquid booster, and im also using seachem flourish.. The API leaf zone im using the amount per the intructions only once a week, the flourish im using 2x a week, and the co2 booster im using everyday..