No you do not need it because crawfish are not good swimers and can die if you have that much water.
I would highly recommend it, yes
It never hurts to have a filter it will only improve your fish's chances of survival and decrease your need to clean.
Air bubbles will do nothing. What is needed is a filter.
It never hurts to have a filter it will only improve your fish's chances of survival and decrease your need to clean.
Freshwater snails do not need a filter, but they prefer it. The more water movement, the cleaner they/their shell stays. I have 3 mystery snails in a 10 gallon and 2.5 gallon tank with a filter and they're doing very well, but I also have 5 baby mystery snails in an ice cream pail until they fully mature. They're doing well, too.
You may be able to use a very small internal filter with a 2.5 gallon tank. You can check with your local fish/pet store and they may be able to show you ones that would work for your tank
Two. They need a min. of 20 gallons each.
Yes, even if you have Common Goldfish because its gonna smell super bad...
You need 4 more cups. If you have three quarts, then you need one more quart. There are two cups in a pint and two pints in a quart. That means there are four cups in a quart. You need four more cups of water to make one gallon.
The answer is 4.
A 75 gallon tank is not large enough to house an Oscar in the long term, however if the situation is temporary a mid to large sized canister filter or very large hang on the back filter would be nessesary. I recommend an Eheim 2217 or Aquaclear 110.
Need to know what horse power you are thinking of. B T W the pump is not the filter. The filter is the large tank.