No matter what we do ,black algae keeps reappearing in my tank .we had everything out, started again with fresh gravel and plants and it's still come back. I have an undergravel filter plus another internal filter running ,the tank is 36/15/18 inches. I have about 20 small fish in there so it's not overloaded.any suggestions would be welcome.

SImply cut out any direct sunlight or reduce the lighting you're providing I'd say .

I only have the light on for about 8 hrs and we've moved the tank so it doesn't get direct sunlight. Good suggestion but...

More plants maybe? They will out-compete the algae for nitrate and other micro-nutrients.

Presumably the algae is very dark green rather than black. Various catfish will eat green algae on plants and the glass.
My tank is a similar size as yours - I have three plecostomus (a pair who are now 7 inches or so, and their one-remaining offspring who's rather smaller) who do a good job controlling algae but there are lots of smaller species

No, it really is black, and I have catfish and other algae eaters. I did have real plants but they were covered in it ,so I have lots of plastic ones. It's an enigma!

I would suspect the under gravel filter I've nevet used one but I've heard people not recomend them. Do you use a syphon to vaccum the gravel when doing water changes?

Also do you ever test the water for nitrate, Ph etc you could have a general water quality problem how are fish behaving?

8hrs of tank light could actually be quite a bit of light on top of whatever natural light the tankgets. General rule green algea means to much light and brown algea means to little.

Id suggest a bit from all above ...

Cut down on the light to just a few hours in the evening when you're home maybe .

Get a few Plecs who LIVE of algae and far better than normal catfish who probably won't even touch it AND disconect or remove the under gravel filter I've never liked them and they're seen as the work of the Devil by many .