Yeah if the fish are in too smaller tank it will generally stress them out and when fish are stressed they often go pale in color and in crampt conditions their activity levels will go down, there are alot of bonus's to getting a bigger tank not only for the fish and water quality but you will also generally enjoy seeing their true characters/personalitys coming out more when they are happy
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You sound like you are on the right track though, as for filter maintanence only clean the filter out in water from water changes as the filter sponge contains beneficial bacteria that break down toxins in the tank water, chlorine in tap water will kill off your beneficial bacteria as will over cleaning the filter sponge, you just want to clean the gunky bits out no more than once every 10days
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It will probably be worth investing in some water quality test kits as well, even the most experienced of fish keepers will own some if they are any good, test kits for ammonia, nitrites and nitrates are the only real way to know if your water quality is good, and water quality can be bad even if the water looks sparkling clean. Knowing whats going on in your water quality is very important if you have fish dying or they aern't looking too well as water quality issues are one of the most common causes for sick fish. Ammonia and nitrites should always be 0 and no more in a tank, nitrates should be kept under 40- things like ph, gh and kh aern't really important to know in comparsion to ammonia, nitrites and nitrates.
If you are upgrading your tank you will need to get a bigger filter to be effective at keeping the tank clean but to avoid the tank cycling(when you lose your beneficial bacteria and have to start a new colony and experince many water quality issues- for more info check out the pinned articles in the beginners section or do a search on the forum), keep the old filter running along side the new one for a week in the new tank while the fish are in the new tank.
Aditional info; tank temp should be around 24/26 degrees for platys preferably like most livebearers, you may need to upgrade the heater if you getting a bigger tank so it is powerful enough to heat the tank without breaking. Hope this info helps
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