Not if you keep the filtration up. Every tank is allowed to be overstocked to some extent. My 10g was overstocked because I had 5 platies, 2 mollies, 3 guppies, a male betta, a school of 5 zebra danios, and 6 kuhli loaches. But, everyone worked out fine! I would say that you just need enough filtration, oxygen, and space in the tank. Make sure that no one is too cramped.
100g Native Fish Pond, 55g Goldfish/Catfish Community, 50g Plant Pond, 40gB Tetra/Cory Community, 20gHM Community, 20gH Turtle (Bender [RES]), 20gL Turtle (Kif [YBS]), 15g Reef, 5.5g Betta (Okami), 5gFBH Betta (Pron), 5gHex Betta (Twister)
2 dogs, 13 cats, 2 turtles, 1 bearded dragon and 1 crested gecko