Yes, platies need a filter. Even though platies are known to be hardy and able to withstand almost any condition, living in dirty and murky water will eventually cause stress, sickness, and even death.
You should install a filter to keep your tank cleaner and your platies healthier. Even though platies don’t excrete a high amount of bioload, waste can quickly accumulate and poison the water.
Aside from eliminating toxins from the water, keeping a filter has many other benefits. These include increasing water aeration, encouraging the growth of beneficial bacteria, and slowing down the algae growth rate.