Most people build a DIY aquarium to keep tropical fish. Even the largest aquarium will not have as much free-flowing fresh water as the fish are usually in, and the warmest home is usually not as warm as their usual environment. Having enough clean water and sufficient heat for the water are the most important things to assure before you get the fish and put them in their new home.
A 10-gallon tank, about the smallest usually recommended, will weigh about 11 pounds empty and 111 pounds when filled. A tank of 20 gallons will weigh around 25 pounds empty and 225 pounds filled. Breeder tanks holding 30 or 40 gallons will weigh 45 to 60 pounds empty and 350 to 450 pounds with water.
Heaters of many sizes are available for aquarium tanks, and the heater size required is determined by how much above room temperature the tank and the fish need to be. Heat differences of 9oF or 5oC, 18oF or 10oC or 27oF (15oC) are commonly required, and to produce these 25, 50 or 75 watt heaters are needed. To heat a 20-gallon tank from 68 to 77o usually requires a 50 watt heater, for example. Larger tanks like 50 or 75 gallons may require two heaters, and a spare heater is always a good idea for when yours burns out.
All the water in the tank should pass through a filter at least 4 times an hour, and a higher flow rate is always better for the fish. A 20-gallon tank would need a flow rate of 80 gallons per hour, and a 100-gallon-per-hour filter would be recommended; likewise, 120 gallons per hour for a 30-gallon tank and preferably a 150-gallon-per-hour filter. The right filter and heater are crucial for any DIY aquarium.