How many fish can I put in my 25 gallon aquarium?

: You could but a few shrimps is all you could keep in there and it would be a very small marine tank .An experienced fish keeper could mange that but i always advice 30 gallons minimum for starting a marine tank .

it doesn't need to go on the tank at all. Put it wherever you want and run air tubing to the item(s) in the aquarium

9.31 US Gallons

if its only the top black plastic piece sure you can , id use superglue and let dry (hours not minutes) and wash it several times with water to clean residue off , (NO fish in tank or water ) if crack is to the top of actual tank < its possible with aquarium silicone but I wouldn't chance it id replace it

How many watts you need depends on the type of plants and the volume of the tank. Here is an excerpt from an article that will help you determine proper wattage.
Instructions

Identify the species of plants you have in your tank or that you plan to put in your tank and how much light that species needs. Each species of plant will have an optimal lighting requirements, from low light to bright light. To determine the plant species, compare photos on the Aquarium Plants Photos and Profiles page of the Aqua Hobby website, or take samples of the aquatic plants to your local aquarium store for identification.

Write down how much lighting each of the plant species in your tank needs. Give each plant a rating of low, medium or bright. The books "The Complete Fishkeeper" and "Freshwater Aquariums for Dummies," as well as your local fish store, are good sources of information for this.

Review your list and determine what type of plant dominates your tank. For example, if you have mostly plants that need bright light, then use a bright-light formula. If you have an equal mix of plant lighting needs, then use a medium-light formula.

Calculate a range of watts of lighting you need for your tank with one of the following formulas, depending on the amount of light you determined you needed for your plants. For low light, multiply one and two by the number of gallons. For medium light, multiply two and three by the number of gallons. For bright light, calculate three and five by the number of gallons. For example, for a 20-gallon tank with plants that need medium light, you would get a range of 40 to 60 watts (2 x 20 = 40 and 3 x 20 = 60).

Read more: How to Determine Watts Per Gallon for Aquarium Lighting ' eHow.com http://www.ehow.com/how_8014286_determine-per-gallon-aquarium-lighting.html#ixzz23KB8HErA

There are some limiting factors like the oxygen availability and the filtration processing. I gave you some examples below they all refer for small or large freshwater fish.

3 cm of adult fish length per 4 liters of water.1 cm of adult fish length per 30 square centimeters of surface area. 1 inch of adult fish length per US gallon of water. 1 inch of adult fish length per 12 square inches of surface area.

If you own a regular fish tank without any plants you only need enough light to be able to see your fish clearly and conveniently. To get that result you'll need about 0.75 watt per gallon.

Live rocks require one to two pounds per gallon, depending on the dimensions of the tank and the density of the rock. I won't suggest you this system if you have a heavily populated fish tank.

Try a local pet store, but from my experience once this starts the best solution is to just get a new tank

Most saltwater aquariums are generally used to housetropical fish. Tropical fish are fromthe Tropics (the area of the planet around the equator.) The water temperature varies in differentlatitudes and depths, but it tends to stay between 77-80F (about25-27C). Therefore, you want to houseyour fish in the environment that is most like the one they are accustomed to,if you want them to stay healthy. Therefore, maintaining aquarium temperature with heating and coolingunits is essential.