Help White worms

[Aquaria]

hello i got a 20 gallon FW tank and am working on cycleing my first SW tank (46G bow). i noticed these white worms when i got home from work today and turned on the light. i was reading they were some kind of flatworm BUT they were free swimming non sticking on the glass or anything all swimming, i say WERE because 20 min later they were ALL gone not on the glass or anything cant see anything any more help i dont want to run a SW tank if i cant take care of a fresh water one help plz P.s. been running for 3 months got 2 giant danios, 2 rosy barbs, 2 oddesa barbs, 1 bala shark, 1 pleco, 3 swordtails, i know its over stocked but there all babys and got about 3 more months or so before i have to get a bigger tank for the pleco, bala and danios. its all under the 1 inch/ gallon rule got about 5 inches of growth left :P although my bala has doubled in size over the last month lol might need one this month or next haha