The java fern in my main tank is starting to go brown, I had 5 really big clumps growing really well and just this week noticed that the leaves are all mushy and disintegrating. Is this normal?

IME it is not normal. Java fern is a fairly hardy plant.

Have your water parameters changed radically over the last week?

Have you been dosing ferts, supplements, carbon?

This happens occasionally, there are a few theories about why but noone's sure. In my experience it most often happens with a big water change after there hasn't been one for a while, or when the plants are first introduced. There's nothing much you can do, except maybe isolating some of the plants that haven't been affected yet. Some of the plants usually survive anyways and repopulate the tank quick enough.

well I did start dosing with PMDD about 3 weeks ago, 10 drops in my 210 litre tank approx 2x weekly but i didnt think it would do this! the other plants are growing really well - eg tiger lotus and red pine

not sure what to do! i removed some brown leaves but its spreading everywhere!

Ive had this a couple of times for no apparant reason usually just leve it and fresh leaves start to come through

I had this happen a while back. When I checked my water, the nitrate and phosphate levels were way too high. So I got some phosphate remover and did a small water change every second day to bring the nitrate and phosphate back down to where they should be, and the Java fern came right. This may or may not be the case with you, so check your water!

ok will do! sounds like water changes might be the way to go. what phosphate remover did u use? the phos-zorb one in a bag?

I think it was JBL's PhosEX what I used, but that should work fine, don't use it unless your phosphate is high though, otherwise it'll be a waste.

are you under strong lighting +

major water change?

fern will "burn" if your water have very big change