Metal Halide in a 25 is a great option. I have seen very successful 20 gallons (long) with only 70 watts but for a 25 standard I would recommend a 150, you could do a 70 but it would be limiting and the cost is the same. Don't go crazy with a 250 watt that'll be overkill, just get a reputable 150 pendant, with a kelvin temperature of about 14,000 so you won't need additional actnics, and make sure it is an HQI design with electronic ballasts (these typically have the best intensity and longest life for MH).