Fry have a reputation for being sucked into filter intakes. The question is, how does this occur and is it actually a problem?
The most often recommended type of filter for the fry tank is the sponge filter. This type of filter works by pulling water through the sponge. The holes in the sponge are small enough and the water current is weak enough that fry and eggs are unharmed and cannot be pulled inside. The microorganisms growing on the sponge are also considered to be a source of food for fry. This makes these filters doubly valuable in the fry tank.
When rearing fry I used a box filter since it was what I happened to have handy. Fry got into the filter, but they seemed completely unharmed by the experience, being healthy and about the same size as fry living outside the filter. I merely check for fry when I clean the filter and move any that I find into the main tank.
Undergravel filters should not pose a problem in drawing fry into themselves as water intake occurs through the gravel bed. I suppose it is potentially possible that eggs could get sucked down and any resulting fry be stuck in the gravel bed, but I could not say if or how often this happens.
I suspect the main complaints about fry being sucked into filters involve power filters where there is quite a strong a current to draw fry into the intake tube. The danger to fry could be reduced by placing something over the filter intake that has very small holes – such as a piece of nylon ladies stocking attached with an elastic band. The holes should be too small for the fry to be sucked through into the filter. However, if current at the intake is extremely strong the filter could still potentially harm the fry by holding them against the nylon stocking by a current of water too strong for them to swim against. I have no personal experience using this sort of filter with fry, but based on other people’s comments I would not recommend it.
Sponge filters are usually very cheap to buy so you may find the best solution for a fry rearing tank is to buy one of them and leave the power filters for tanks with adult fish. A simple sponge filter may cost around $5-10 CAN, or it is possible to make your own. However, in addition to the filter you will also need plastic tubing and an air pump. An air pump for a small tank is usually under $20.00 CAN, but you may already have one so check before buying. With a little extra tubing and valves you can often hook more than one thing up to the same air pump.
In short, fry getting into the filter is only a problem with certain filter types. With box, undergravel, or sponge filters fry either won’t enter the filter, or are unlikely to be harmed if they do.