Hi, it sounds like your plants are rotting, dirtying up the water and creating algae. What's worked for me is three-fold:
I cut away dead parts and removed plants that were dead to the bottom. New ones may need to be bought if it looks too bare, but that rotting dead stuff's gotta go.
I do a 1/3 water change once a week. Purified water, left to sit out in a big container for 24 hours; I put an aquarium thermometer in there and heat a bit of the water so that water temp matches the tank's temp within 2-3 degrees when I pour in slowly. The filter needs to be replaced since the tank is so dirty. After this overhaul, rinsing filter at the time of the water change each week will leave it in good shape for 2-3 months before it needs to be replaced again.
This won't scare the fish if they "own" their space. They may even be intensely curious and dog the action as your hand moves around and the new water is dipped out/poured in. Anyone who's scared knows enough to go hide in bed until it's over. No trauma.
Finally, I invested $10 in three Nerite snails to clean up the algae after doing all of the above. I'd been wiping the glass and accomplishing very little, which annoyed the plankton out of me. I figured they'd get some of it, after an adjustment period. Nope. The little devils vacuumed every spot off the glass in the first 24 hours. Then they set to work on the plants, accessories, shell decor, everything. The tank was spotless within days. They're little and get into nooks and crannies. Plus they're pretty, zebra stripes. And they're peaceful and keep to themselves.
It seems like a lot but if you bite the bullet and do the big overhaul, upkeep is painless. It takes 10 minutes every Sunday morning for me to rinse the filter, squeeze out the moss ball, dip out old water/pour in new, and trim out any newly browning plant parts. Afterwards my fish, in his crystal-clear water, darts around happily inspecting everything as though he's the one who did all the work.