Originally posted by my03
first of all, how much impedence will glass lids have on the lighting provided by M/h or flurecent tubes?
If the glass is kept clean, somewhere around 5%-10% of light will be blocked, depending on the thickness of glass and wether or not its tempered. Acrylic sheets will allow more light through, but are more expensive, need to be thicker to resist warping, and tend to scratch.
If the glass is not frequently cleaned of all salt or any other build up, it will drastically reduce light transmission, by 50% or more.
how serious is the issue of evaporation creaing higher salinity levels?
Well, open tops most certainly increase your evaporation, the increase in air exchange will force you to add larger amounts of freshwater to compensate each day, but the benefits generally outweigh the negatives.
what are ways to overcome this? is this what "chillers" are for?
chillers do exactly what the name conveys, they cool down the tank water, much the same way your refrigerator works. Many people find that using an open top allows them to get by
without a chiller, as the additional evaporation cools the tank water.
BTW- if you do decide to go with an open top, consider buying a sheet or two of eggcrate (plastic light diffuser grid) many of us use that to prevent the occasional fish from carpet surfing.
