I have a GS500E but it is having exceloration problems.
i have cleaned tank, fuel was brown, put new fuel in  and still doing problem drained tank again, fuel still brown.
any reason why.

Have you looked inside the tank to see if its rusty? You way not be able to see down in to the rust pockets at the bottom of the tank though.

You could try putting in an in-line fuel filter to see if that is a sufficient bandage for the issue. If the rust is real bad it will quickly clog an external fuel filter though.

If rust is the problem the only solution is to de-rust the tank using an off-the-shelf product or spend a day shaking the tank with white vinegar + a handfull of nuts and bolts. Once you de-rust the tank you can try an epoxy tank liner to seal it. People have mixed results with the tank liners though.

Ive had this with many GS500s here in the uk. Had mixed results with tank sealer. best one so far was the Tappox system where you convert the rust then seal the tank. secret is to get fresh air into the tank during the curing process. I use a 12v computer server fan aprox 50mm diameter taped onto a length of plastic pipe around 400mm long. The fan blows into the tank (NOT SUCKING AIR OUT) as this will send the solvents through the fan. Not a good Idea having the fumes so close to  the electrical action of the fan. (Fuel tap removed to let the air out of the bottom.) You can leave this running for around 2 to 3 days ensuring a proper cure. (This is where most systems fail)
Alternatively you can purchase a decent used tank as its quicker and easier.
Most common cause is water getting into the tank and sitting at the bottom happily eating away at the surface. The water gets in usually because the drain pipe which runs through the tank itself becomes blocked. When it rains or when you wash your bike the water builds up in the well under your fuel cap and runs in past the seal on the fuel cap. Around 40% of the bikes I work on here in the uk have water in the tank!
Regularly remove your fuel cap and shroud. Take a piece of wire and clean the drain tube or use an airline to blow it through. saves a lot of heartache I can tell you.